FOOTNOTES:
[148] For these paragraphs on the history of the study of fossil fishes the writer is indebted to the kind interest of Professor Bashford Dean.
[149] Dr. Arthur Smith Woodward excepted.
[CHAPTER XXIII]
THE COLLECTION OF FISHES
How to Secure Fishes.—In collecting fishes three things are vitally necessary—a keen eye, some skill in adapting means to ends, and some willingness to take pains in the preservation of material.
In coming into a new district the collector should try to preserve the first specimen of every species he sees. It may not come up again. He should watch carefully for specimens which look just a little different from their fellows, especially for those which are duller, less striking, or with lower fins. Many species have remained unnoticed through generations of collectors who have chosen the handsomest or most ornate specimens. In some groups with striking peculiarities, as the trunkfishes, practically all the species were known to Linnæus. No collector could pass them by. On the other hand, new gobies or blennies can be picked up almost every day in the lesser known parts of the world. For these overlooked forms—herrings, anchovies, sculpins, blennies, gobies, scorpion-fishes—the competent collector should be always on the watch. If any specimen looks different from the rest, take it at once and find out the reason why.
In most regions the chief dependence of the collector is on the markets and these should be watched most critically. By paying a little more for unusual, neglected, or useless fish, the supply of these will rise to the demand. The word passed along among the people of Onomichi in Japan, that "Ebisu the fish-god was in the village" and would pay more for okose (poison scorpion-fishes) and umiuma (sea-horses) than real fishes were worth soon brought (in 1900) all sorts of okose and umiuma into the market when they were formerly left neglected on the beach. Thus with a little ingenuity the markets in any country can be greatly extended.
The collector can, if he thinks best, use all kinds of fishing tackle for himself. In Japan he can use the "dabonawa" long lines, and secure the fishes which were otherwise dredged by the Challenger and Albatross. If dredges or trawls are at his hand he can hire them and use them for scientific purposes. He should neglect no kind of bottom, no conditions of fish life which he can reach.
Especially important is the fauna of the tide-pools, neglected by almost all collectors. As the tide goes down, especially on rocky capes which project into the sea, myriads of little fishes will remain in the rock-pools, the algæ, and the clefts of rock. In regions like California, where the rocks are buried with kelp, blennies will lie in the kelp as quiescent as the branches of the algæ themselves until the flow of water returns.
A sharp three-tined fork will help in spearing them. The water in pools can be poisoned on the coast of Mexico with the milky juice of the "hava" tree, a tree which yields strychnine. In default of this, pools can be poisoned by chloride of lime, sulphate of copper, or, if small enough, by formaline. Of all poisons the commercial chloride of lime seems to be most effective. By such means the contents of the pool can be secured and the next tide carries away the poison. The water in pools can be bailed out, or, better, emptied by a siphon made of small garden-hose or rubber tubing. On rocky shores, dynamite can be used to advantage if the collector or his assistant dare risk it and if the laws of the country do not prevent.
Most effective in rock-pool work is the help of the small boy. In all lands the collector will do well to take him into his pay and confidence. Of the hundred or more new species of rock-pool fishes lately secured by the writer in Japan, fully two-thirds were obtained by the Japanese boys. Equally effective is the "muchacho" on the coasts of Mexico.
Masses of coral, sponges, tunicates, and other porous or hollow organisms often contain small fishes and should be carefully examined. On the coral reefs the breaking up of large masses is often most remunerative.
The importance of securing the young of pelagic fishes by tow-nets and otherwise cannot be too strongly emphasized.
How to Preserve Fishes.—Fishes must be permanently preserved in alcohol. Dried skins are far from satisfactory, except as a choice of difficulties in the case of large species.
Dr. Günther thus describes the process of skinning fishes:
"Scaly fishes are skinned thus: With a strong pair of scissors an incision is made along the median line of the abdomen from the foremost part of the throat, passing on one side of the base of the ventral and anal fins to the root of the caudal fin, the cut, being continued upward to the back of the tail close to the base of the caudal. The skin of one side of the fish is then severed with the scalpel from the underlying muscles to the median line of the back; the bones which support the dorsal and caudal are cut through, so that these fins remain attached to the skin. The removal of the skin of the opposite side is easy. More difficult is the preparation of the head and scapulary region. The two halves of the scapular arch which have been severed from each other by the first incision are pressed toward the right and left, and the spine is severed behind the head, so that now only the head and shoulder bones remain attached to the skin. These parts have to be cleaned from the inside, all soft parts, the branchial and hyoid apparatus, and all smaller bones being cut away with the scissors or scraped off with the scalpel. In many fishes which are provided with a characteristic dental apparatus in the pharynx (Labroids, Cyprinoids), the pharyngeal bones ought to be preserved and tied with a thread to their specimen. The skin being now prepared so far, its entire inner surface as well as the inner side of the head are rubbed with arsenical soap; cotton-wool or some other soft material is inserted into any cavities or hollows, and finally a thin layer of the same material is placed between the two flaps of the skin. The specimen is then dried under a slight weight to keep it from shrinking.
"The scales of some fishes, as for instance of many kinds of herrings, are so delicate and deciduous that the mere handling causes them to rub off easily. Such fishes may be covered with thin-paper (tissue paper is the best) which is allowed to dry on them before skinning. There is no need for removing the paper before the specimen has reached its destination.
"Scaleless fishes, as siluroids and sturgeons, are skinned in the same manner, but the skin can be rolled up over the head; such skins can also be preserved in spirits, in which case the traveler may save to himself the trouble of cleaning the head.
"Some sharks are known to attain to a length of thirty feet, and some rays to a width of twenty feet. The preservation of such gigantic specimens is much to be recommended, and although the difficulties of preserving fishes increase with their size, the operation is facilitated, because the skins of all sharks and rays can easily be preserved in salt and strong brine. Sharks are skinned much in the same way as ordinary fishes. In rays an incision is made not only from the snout to the end of the fleshy part of the tail, but also a second across the widest part of the body. When the skin is removed from the fish, it is placed into a cask with strong brine mixed with alum, the head occupying the upper part of the cask; this is necessary, because this part is most likely to show signs of decomposition, and therefore most requires supervision. When the preserving fluid has become decidedly weaker from the extracted blood and water, it is thrown away and replaced by fresh brine. After a week's or fortnight's soaking the skin is taken out of the cask to allow the fluid to drain off; its inner side is covered with a thin layer of salt, and after being rolled up (the head being inside) it is packed in a cask the bottom of which is covered with salt; all the interstices and the top are likewise filled with salt. The cask must be perfectly water-tight."
Value of Formalin.—In the field it is much better to use formalin (formaldehyde) in preference to alcohol. This is an antiseptic fluid dissolved in water, and it at once arrests decay, leaving the specimen as though preserved in water. If left too long in formalin fishes swell, the bones are softened, and the specimens become brittle or even worthless. But for ordinary purposes (except use as skeleton) no harm arises from two or three months' saturation in formalin. The commercial formalin can be mixed with about twenty parts of water. On the whole it is better to have the solution too weak rather than too strong. Too much formalin makes the specimens stiff, swollen, and intractable, besides too soon destroying the color.
Formalin has the advantage, in collecting, of cheapness and of ease in transportation, as a single small bottle will make a large amount of the fluid. The specimens also require much less attention. An incision should be made in the (right) side of the abdomen to let in the fluid. The specimen can then be placed in formalin. When saturated, in the course of the day, it can be wrapped in a cloth, packed in an empty petroleum can, and at once shipped. The wide use of petroleum in all parts of the world is a great boon to the naturalist.
Before preservation, the fishes should be washed, to remove slime and dirt. They should have an incision to let the fluid into the body cavity and an injection with a syringe is a useful help to saturation, especially with large fishes. Even decaying fishes can be saved with formalin.
Records of Fishes.—The collector should mark localities most carefully with tin tags and note-book records if possible. He should, so far as possible, keep records of life colors, and water-color sketches are of great assistance in this matter. In spirits or formalin the life colors soon fade, although the pattern of marking is usually preserved or at least indicated. A mixture of formalin and alcohol is favorable to the preservation of markings.
In the museum all specimens should be removed at once from formalin to alcohol. No substitute for alcohol as a permanent preservative has been found. The spirits derived from wine, grain, or sugar is much preferable to the poisonous methyl or wood alcohol.
In placing specimens directly into alcohol, care should be taken not to crowd them too much. The fish yields water which dilutes the spirit. For the same reason, spirits too dilute are ineffective. On the other hand, delicate fishes put into very strong alcohol are likely to shrivel, a condition which may prevent an accurate study of their fins or other structures. It is usually necessary to change a fish from the first alcohol used as a bath into stronger alcohol in the course of a few days, the time depending on the closeness with which fishes are packed. In the tropics, fishes in alcohol often require attention within a few hours. In formalin there is much less difficulty with tropical fishes.
Fishes intended for skeletons should never be placed in formalin. A softening of the bones which prevents future exact studies of the bones is sure to take place. Generally alcohol or other spirits (arrack, brandy, cognac, rum, sake "vino") can be tested with a match. If sufficiently concentrated to be ignited, they can be safely used for preservation of fishes. The best test is that of the hydrometer. Spirits for permanent use should show on the hydrometer 40 to 60 above proof. Decaying specimens show it by color and smell and the collector should be alive to their condition. One rotting fish may endanger many others. With alcohol it is necessary to take especial pains to ensure immediate saturation. Deep cuts should be made into the muscles of large fishes as well as into the body cavity. Sometimes a small distilling apparatus is useful to redistil impure or dilute alcohol. The use of formalin avoids this necessity.
Small fishes should not be packed with large ones; small bottles are very desirable for their preservation. All spinous or scaly fishes should be so wrapped in cotton muslin as to prevent all friction.
Eternal Vigilance.—The methods of treating individual groups of fishes and of handling them under different climatic and other conditions are matters to be learned by experience. Eternal vigilance is the price of a good collection, as it is said to be of some other good things. Mechanical collecting—picking up the thing got without effort and putting it in alcohol without further thought—rarely serves any useful end in science. The best collectors are usually the best naturalists. The collections made by the men who are to study them and who are competent to do so are the ones which most help the progress of ichthyology. The student of a group of fishes misses half the collection teaches if he has made no part of it himself.
[CHAPTER XXIV]
THE EVOLUTION OF FISHES
The Geological Distribution of Fishes.—The oldest unquestioned remains of fishes have been very recently made known by Mr. Charles D. Walcott, from rocks of the Trenton period in the Ordovician or Lower Silurian. These are from Cañon City in Colorado. Among these is certainly a small Ostracophore (Asteraspis desideratus). With it are fragments (Dictyorhabdus) thought to be the back-bone of a Chimæra, but more likely, in Dean's view, the axis of a cephalopod, besides bony, wrinkled scales, referred with doubt to a supposed Crossopterygian genus called Eriptychius. This renders certain the existence of Ostracophores at this early period, but their association with Chimæras and Crossopterygians is questionable. Primitive sharks may have existed in Ordovician times, but thus far no trace of them has been found.
Fig. 246.—Fragment of Sandstone from Ordovician deposits, Cañon City, Colo., showing fragments of scales, etc., the earliest known traces of vertebrates. (From nature.)
The fish-remains next in age in America are from the Bloomfield sandstone in Pennsylvania of the Onondaga period in the upper Silurian. The earliest in Europe are found in the Ludlow shales, both of these localities being in or near the horizon of the Niagara rocks, in the Upper Silurian Age.
It is, however, certain that these Lower Silurian remains do not represent the beginning of fish-life. Probably Ostracophores, and Arthrodires, with perhaps Crossopterygians and Dipnoans, existed at an earlier period, together perhaps with unarmed, limbless forms without jaws, of which no trace whatever has been left.
Fig. 247.—Fossil fish remains from Ordovician rocks, Cañon City, Colo. (After Walcott.) a. Scale of Eriptychius americanus Walcott. Family Holoptychiidæ? b. Dermal plate of Asteraspis desideratus Walcott. Family Asterolepidæ. c. Dictyorhabdus priscus Walcott, a fragment of uncertain nature, thought to be a chordal sheath of a Chimæra, but probably part of a Cephalopod (Dean). Chimæridæ?
The Earliest Sharks.—The first actual trace of sharks is found in the Upper Silurian in the form of fin-spines (Onchus), thought to belong to primitive sharks, perhaps Acanthodeans possibly to Ostracophores. With these are numerous bony shields of the mailed Ostracophores, and somewhat later those of the more highly specialized Arthrodires. Later appear the teeth of Cochliodontidæ, with Chimæras, a few Dipnoans, and Crossopterygians.
Devonian Fishes.—In the Devonian Age the Ostracophores increase in size and abundance, disappearing with the beginning of the Carboniferous. The Arthrodires also increase greatly in variety and in size, reaching their culmination in the Devonian, but not disappearing entirely until well in the Carboniferous. These two groups (often united by geologists under the older name Placoderms) together with sharks and a few Chimæras made up almost exclusively the rich fish-fauna of Devonian times. The sharks were chiefly Acanthodean and Psammodont, as far as our records show. The supposed more primitive type of Cladoselache is not known to appear before the latter part of the Devonian Age, while Pleuracanthus and Cladodus, sometimes regarded as still more primitive, are as yet found only in the Carboniferous. It is clear that the records of early shark life are still incomplete, whatever view we may adopt as to the relative rank of the different forms. Chimæroids occur in the Devonian, and with them a considerable variety of Crossopterygians and Dipnoans. The true fishes appear also in the Devonian in the guise of the Ganoid ancestors and relatives of Palæoniscum, all with diamond-shaped enameled scales. In the Devonian, too, we find the minute creature Palæospondylus, our ignorance of which is concealed under the name Cycliæ.
Carboniferous Fishes.—In the Carboniferous Age the sharks increase in number and variety, the Ostracophores disappear, and the Arthrodires follow them soon after, the last being recorded from the Permian. Other forms of Dipnoans, Crossopterygians, and some Ganoids now appear giving the fauna a somewhat more modern aspect. The Acanthodei and the Ichthyotomi pass away with the Permian, the latest period of the Carboniferous Age.
Fig. 248.—Dipterus valenciennesi Agassiz, a Dipnoan. (After Dean, from Woodward.)
Mesozoic Fishes.—In the Triassic period which follows the Permian, the earliest types of Ganoids give place to forms approaching the garpike and sturgeon. The Crossopterygians rapidly decline. The Dipnoans are less varied and fewer in number; the primitive sharks, with the exception of certain Cestracionts, all disappear, only the family of Orodontidæ remaining. Here are found the first true bony fishes, doubtless derived from Ganoid stock, the allies and predecessors of the great group of herrings. Herring-like forms become more numerous in the Jurassic, and with them appear other forms which give the fish-fauna of this period something of a modern appearance. In the Jurassic the sharks become divided into several groups, Notidani, Scyllioid sharks, Lamnoid sharks, angel-fishes, skates, and finally Carcharioid sharks being now well differentiated. Chimæras are still numerous. The Acanthodei have passed away, as well as the mailed Ostrachopores and Arthrodires. The Dipnoans and Crossopterygians are few. The early Ganoids have given place to more modern types, still in great abundance and variety. This condition continues in the Cretaceous period. Here the rays and modern sharks increase in number, the Ganoids hold their own, and the other groups of soft-rayed fishes, as the smelts, the lantern-fishes, the pikes, the flying-fishes, the berycoids and the mackerels join the group of herring-like forms which represent the modern bony fishes. In the Cretaceous appear the first spiny-rayed fishes, derived probably from herring-like forms. These are allies or ancestors of the living genus Beryx.
Fig. 249.—Hoplopteryx lewesiensis (Mantell), restored. English Cretaceous. Family Berycidæ. (After Woodward.)
Dr. Woodward observes:
"As soon as fishes with a completely osseous endoskeleton began to predominate at the dawn of the Cretaceous period, specializations of an entirely new kind were rapidly acquired. Until this time the skull of the Actinopterygii had always been remarkably uniform in type. The otic region of the cranium often remained incompletely ossified and was never prominent or projecting beyond the roof bones; the supraoccipital bone was always small and covered with the superficial plates; the maxilla invariably formed the greater part of the upper jaw; the cheek-plates were large and usually thick; while none of the head or opercular bones were provided with spines or ridges. The pelvic fins always retained their primitive remote situation, and the fin-rays never became spines. During the Cretaceous period the majority of the bony fishes began to exhibit modifications in all these characters, and the changes occurred so rapidly that by the dawn of the Eocene period the diversity observable in the dominant fish-fauna was much greater than it had ever been before. At this remote period, indeed, nearly all the great groups of bony fishes, as represented in the existing world, were already differentiated, and their subsequent modifications have been quite of a minor character."
Fig. 250.—A living Berycoid fish, Paratrachichthys prosthemius Jordan & Fowler. Misaki, Japan. Family Berycidæ.
Fig. 251.—Flying-fish, Cypsilurus heterurus (Rafinesque). Family Exocætidæ Woods Hole, Mass.
Fig. 252.—The Schoolmaster Snapper, a Perch-like fish. Family Lutianidæ. Key West.
Tertiary Fishes.—With the Eocene or first period of the Tertiary great changes have taken place. The early families of bony fishes nearly all disappear. The herring, pike, smelt, salmon, flying-fish, and berycoids remain, and a multitude of other forms seem to spring into sudden existence. Among these are the globefishes, the trigger-fishes, the catfishes, the eels, the morays, the butterfly-fishes, the porgies, the perch, the bass, the pipefishes, the trumpet-fishes, the mackerels, and the John-dories, with the sculpins, the anglers, the flounders, the blennies, and the cods. That all these groups, generalized and specialized, arose at once is impossible, although all seem to date from the Eocene times. Doubtless each of them had its origin at an earlier period, and the simultaneous appearance is related to the fact of the thorough study of the Eocene shales, which have in numerous localities (London, Monte Bolca, Licata, Mount Lebanon, Green River) been especially favorable for the preservation of these forms. Practically fossil fishes have been thoroughly studied as yet only in a very few parts of the earth. The rocks of Scotland, England, Germany, Italy, Switzerland, Syria, Ohio, and Wyoming have furnished the great bulk of all the fish remains in existence. In some regions perhaps collections will be made which will give us a more just conception of the origin of the different groups of bony fishes. We can now only say with certainty that the modern families were largely existent in the Eocene, that they sprang from ganoid stock found in the Triassic and Jurassic, that several of them were represented in the Cretaceous also, that the Berycoids were earliest of the spiny-rayed fishes, and forms allied to herring the earliest of the soft-rayed forms. Few modern families arose before the Cretaceous. Few of the modern genera go back to the Eocene, many of them arose in the Miocene, and few species have come down to us from rocks older than the end of the Pliocene. The general modern type of the fish-faunas being determined in the latter Eocene and the Miocene, the changes which bring us to recent times have largely concerned the abundance and variety of the individual species. From geological distribution we have arising the varied problems of geographical distribution and the still more complex conditions on which depend the extinction of species and of types.
Fig. 253.—Decurrent Flounder, Pleuronichthys decurrens Jordan & Gilbert. San Francisco.
Factors of Extinction.—These factors of extinction have been recently formulated as follows by Professor Herbert Osborn. He considers the process of extinction as of five different types:
"(1) That extinction which comes from modification or progressive evolution, a relegation to the past as a result of the transmutation into more advanced forms. (2) Extinction from changes of physical environment which outrun the powers of adaptation. (3) The extinction which results from competition. (4) The extinction which results from extreme specialization and limitation to special conditions the loss of which means extinction. (5) Extinction as a result of exhaustion."
Fossilization of a Fish.—When a fish dies he leaves no friends. His body is at once attacked by hundreds of creatures ranging from the one-celled protozoa and bacteria to individuals of his own species. His flesh is devoured, his bones are scattered, the gelatinous substance in them decays, and the phosphate of lime is in time dissolved in the water. For this reason few fishes of the millions which die each year leave any trace for future preservation. At the most a few teeth, a fin-spine, or a bone buried in the clay might remain intact or in such condition as to be recognized.
But now and then it happens that a dead fish may fall in more fortunate conditions. On a sea bottom of fine clay the bones, or even the whole body, may be buried in such a way as to be sealed up and protected from total decomposition. The flesh will usually disappear and leave no mark or at the most a mere cast of its surface. But the hard parts, even the muscles may persist, and now and then they do persist, the salts of lime unchanged or else silicified or subjected to some other form of chemical substitution. Only the scales, the teeth, the bones, the spines, and the fin-rays can be preserved in the rocks of sea or lake bottom. In a few localities, as near Green River in Wyoming, Monte Bolca, near Verona, and Mount Lebanon in Syria, the London clays, with certain quarries in Scotland and lithographic stones in Germany, many skeletons of fishes have been found pressed flat in layers of very fine rock, their structures traced as delicately as if actually drawn on the smooth stone. Fragments preserved in ruder fashion abound in the clays and even the sandstones of the earliest geologic ages. In most cases, however, fossil fishes are known from detached and scattered fragments, many of them, especially of the sharks, by the teeth alone. Fishes have occurred in all ages from the Silurian to the present time and probably the very first lived long before the Silurian.
The Earliest Fishes.—No one can say what the earliest fishes were like, nor do we know what was their real relation to the worm-like forms among which men have sought their presumable ancestors, nor to the Tunicates and other chordate forms, not fish-like, but still degenerate relatives of the primeval fish.
From analogy we may suppose that the first fishes which ever were bore some resemblance to the lancelet, for that is a fish-like creature with every structure reduced to the lowest terms. But as the lancelet has no hard parts, no bones, nor teeth, nor scales, nor fins, no traces of its kind are found among the fossils. If the primitive fish was like it in important respects, all record of this has probably vanished from the earth.
The Cyclostomes.—The next group of living fishes, the Cyclostomes, including the hagfishes and lampreys,—fishes with small skull and brain but without limbs or jaws,—stands at a great distance above the lancelet in complexity of structure, and equally far from the true fishes in its primitive simplicity. In fact the lamprey is farther from the true fish in structure than a perch is from an eagle. Yet for all that it may be an offshoot from the primitive line of fish descent. There is not much in the structure of the lamprey which may be preserved in the rocks. But the cartilaginous skull, the back-bone, fins, and teeth might leave their traces in soft clay or lithographic stone. But it is certain that they have not done so in any rocks yet explored, and it may be that the few existing lampreys owe their form and structure to a process of degradation from a more complex and more fish-like ancestry. The supposed lamprey fossil of the Devonian of Scotland, Palæospondylus, has little in common with the true lampreys.
The Ostracophores.—Besides the lampreys the Devonian seas swarmed with mysterious creatures covered with an armor of plate, fish-like in some regards, but limbless, without true jaws and very different from the true fishes of to-day. These are called Ostracophori, and some have regarded them as mailed lampreys, but they are more likely to be a degenerate or eccentric offshoot from the sharks, as highly modified or specialized lampreys, a side offshoot which has left no descendants among recent forms. Recently Professor Patten has insisted that the resemblance of their head-plates to those of the horseshoe crab (Limulus) is indicative of real affinity.
Among these forms in mail-armor are some in which the jointed and movable angles of the head suggest the pectoral spines of some catfishes. But in spite of its resemblance to a fin, the spine in Pterichthyodes is an outgrowth of the ossified skin and has no more homology with the spines of fishes than the mailed plates have with the bones of a fish's cranium. In none of these fishes has any trace of an internal skeleton been found. It must have retained its primitive gelatinous character. There are, however, some traces of eyes, and the mucous channels of the lateral line indicate that these creatures possessed some other special senses.
Fig. 254.—An Ostracophore, Cephalaspis lyelli Agassiz, restored. Devonian. (After Agassiz, per Dean.)
Whatever the Ostracophores may be, they should not be included within the much-abused term Ganoidei, a word which was once used in the widest fashion for all sorts of mailed fishes, but little by little restricted to the hard-scaled relatives and ancestors of the garpike of to-day.
The Arthrodires.—Dimly seen in the vast darkness of Paleozoic time are the huge creatures known as Arthrodires. These are mailed and helmeted fishes, limbless so far as we know, but with sharp, notched, turtle-like jaws quite different from those of the fish or those of any animal alive to-day. These creatures appear in Silurian rocks and are especially abundant in the fossil beds of Ohio, where Newberry, Claypole, Eastman, Dean and others have patiently studied the broken fragments of their armor. Most of them have a great casque on the head with a shield at the neck and a movable joint connecting the two. Among them was almost every variation in size and form.
Fig. 255.—An Arthrodire, Dinichthys intermedius Newberry, restored. Devonian, Ohio. (Family after Dean.)
These creatures have been often called ganoids, but with the true ganoids like the garpike they have seemingly nothing in common. They are also different from the Ostracophores. To regard them with Woodward as derived from ancestral Dipnoans is to give a possible guess as to their origin, and a very unsatisfactory guess at that. In any event these have all passed away in competition with the scaly fishes and sharks of later evolution, and it seems certain that they, like the mailed Ostracophores, have left no descendants.
The Sharks.—Next after the lampreys, but a long way after them in structure, come the sharks. With the sharks appear for the first time true limbs and the lower jaw. The upper jaw is, however, formed from the palate, and the shoulder-girdle is attached behind the skull. "Little is known," says Professor Dean, "of the primitive stem of the sharks, and even the lines of descent of the different members of the group can only be generally suggested. The development of recent forms has yielded few results of undoubted value to the phylogenist. It would appear as if paleontology alone could solve the puzzles of their descent."
Of the very earliest sharks in the Upper Silurian Age the remains are too scanty to prove much save that there were sharks in abundance and variety. Spines, teeth, fragments of shagreen, show that in some regards these forms were highly specialized. In the Carboniferous Age the sharks became highly varied and extensively specialized. Of the Paleozoic types, however, all but a single family seems to have died out, leaving Cestraciontes only in the Permian and Triassic. From these the modern sharks one and all may very likely have descended.
Origin of the Sharks.—Perhaps the sharks are developed from the still more primitive shark imagined as without limbs and with the teeth slowly formed from modification of the ordinary shagreen prickles. In determining the earliest among the several primitive types of shark actually known we are stopped by an undetermined question of theory. What is the origin of paired limbs? Are these formed, like the unpaired fins, from the breaking up of a continuous fold of skin, in accordance with the view of Balfour and others? Or is the primitive limb, as supposed by Gegenbaur, a modification of the bony gill-arch? Or again, as supposed by Kerr, is it a modification of the hard axis of an external gill?
If we adopt the views of Gegenbaur or Kerr, the earliest type of limb is the jointed archipterygium, a series of consecutive rounded cartilaginous elements with a fringe of rays along its length. Sharks possessing this form of limb (Ichthyotomi) appear in the Carboniferous rocks, but are not known earlier. It may be that from these the Dipnoans, on the one hand, may be descended and, on the other, the true sharks and the Chimæras; but there is no certainty that the jointed arm or archipterygium of the Dipnoans is derived from the similar pectoral fin of the Ichthyotomi.
On the other hand, if we regard the paired fins as parts of a lateral fold of skin, we find primitive sharks to bear out our conclusions. In Cladoselache of the Upper Devonian, the pectoral and the ventral fins are long and low, and arranged just as they might be if Balfour's theory were true. Acanthoessus, with a spine in each paired fin and no other rays, might be a specialization of this type or fin, and Climatius, with rows of spines in place of pectorals and ventrals, might be held to bear out the same idea. In all these the tail is less primitive than in the Ichthyotomi. On the other hand, the vent in Cladoselache is thought by Dean to have been near the end of the tail. If this is the case, it should indicate a very primitive character. On the whole, though there is much to be said in favor of the primitive nature of the Ichthyotomi (Pleuracanthus) with the tapering tail and jointed pectoral fin of a dipnoan, and other traits of a shark, yet, on the whole, Cladoselache is probably nearer the origin of the shark-like forms.
The relatively primitive sharks called Notidani have the weakly ossified vertebræ joined together in pairs and there are six or seven gill-openings. This group has persisted to our day, the frilled shark (Chlamydoselachus) and the genera Hexanchus and Heptranchias still showing its archaic characters.
Here the sharks diverge into two groups, the one with the vertebræ better developed and its calcareous matter arranged star-fashion. This forms Hasse's group of Asterospondyli, the typical sharks. The earliest forms (Orodontidæ, Heterodontidæ) approach the Notidani, and so far as geological records go, precede all the other modern sharks. One such ancient type, Heterodontus, including the bullhead shark, and the Port Jackson shark, still persists. The others diverge to form the three chief groups of the cat-sharks (Scyliorhinus, etc.), the mackerel-sharks (Lamna, etc.), and the true sharks (Carcharhias, etc.).
Fig. 256.—Mackerel-shark or Salmon-shark, Lamna cornubica (Gmelin). Santa Barbara, Cal.
In the second group the vertebræ have their calcareous matter arranged in rings, one or more about the notochordal center. In all these the anal fin is absent, and in the process of specialization the shark gradually gives place to the flattened body and broad fins of the ray. This group is called Tectospondyli. Those sharks of this group with one ring of calcareous matter in each vertebra constitute the most primitive extreme of a group representing continuous evolution.
From Cladoselache and Chlamydoselachus through the sharks to the rays we have an almost continuous series which reaches its highest development in the devil rays or mantas of the tropical seas, Manta and Mobula being the most specialized genera and among the very largest of the fishes. However different the rays and skates may appear in form and habit, they are structurally similar to the sharks and have sprung from the main shark stem.
Fig. 257.—Star-spined Ray, Raja stellulata Jordan & Gilbert. Monterey, Cal.
The Chimæras.—The most ancient offshoot from the shark stem, perhaps dating from Silurian times and possibly separated at a period earlier than the date of any known shark, is the group of Holocephali or Chimæras, shark-like in essentials, but differing widely in details. Of these there are but few living forms and the fossil types are known only from dental plates and fin-spines. The living forms are found in the deeper seas the world over, one of the simplest in structure being the newly discovered Rhinochimæra of Japan. The fusion of the teeth into overlapping plates, the covering of the gills by a dermal flap, the complete union of the palato-quadrate apparatus or upper jaw with the skull and the development of a peculiar clasping spine on the forehead of the male are characteristic of the Chimæras. The group is one of the most ancient, but it ends with itself, none of the modern fishes being derived from Chimæras.
Fig. 258.—A Deep-sea Chimæra, Harriotta raleighiana Goode & Bean. Gulf Stream.
Fig. 259.—An extinct Dipnoan, Dipterus valenciennesi Agassiz. Devonian. (After Pander.)
The Dipnoans.—The most important offshoot of the primitive sharks is not the Chimæras, nor even the shark series itself, but the groups of Crossopterygians and Dipnoans, or lung-fishes, with the long chain of their descendants. With the Dipnoan appears the lung or air-bladder, at first an outgrowth from the ventral side of the œsophagus, as it still is in all higher animals, but later turning over, among fishes, and springing from the dorsal side. At first an arrangement for breathing air, a sort of accessory gill, it becomes the sole organs of respiration in the higher forms, while in the bony fishes its respiratory function is lost altogether. The air-bladder is a degenerate lung. In the Dipnoans the shoulder-girdle moves forward to the skull, and the pectoral limb, a jointed and fringed archipterygium, is its characteristic appendage. The shark-like structure of the mouth remains.
The few living lung-fishes resemble the salamanders in many regards, and some writers have ranged the class as midway between the primitive sharks and the amphibians. These forms show their intermediate characters in the development of lungs and in the primitive character of the pectoral and ventral limbs. Those now extant give but little idea of the great variety of extinct Dipnoans. The living genera are three in number—Neoceratodus in Australian rivers, Lepidosiren in the Amazon, and Protopterus in the Nile. These are all mudfishes, some of them living through most of the dry season encased in a cocoon of dried mud. Of these forms Neoceratodus is certainly the nearest to the ancient forms, but its embryology, owing to the shortening of its growth stages due to its environment, has thrown little light on the question of its ancestry.
From some ally of the Dipnoans the ancestry of the amphibians, and through them that of the reptiles, birds, and mammals may be traced, although a good deal of evidence has been produced in favor of regarding the primitive crossopterygian or fringe fin as the point of divergence. It is not unlikely that the Crossopterygian gave rise to Amphibian and Dipnoan alike.
In the process of development we next reach the characteristic fish mouth in which the upper jaw is formed of maxillary and premaxillary elements distinct from the skull. The upper jaw of the shark is part of the palate, the palate being fused with the quadrate bone which supports the lower jaw. That of the Dipnoan is much the same. The development of a typical fish mouth is the next step in evolution, and with its appearance we note the decline of the air-bladder in size and function.
The Crossopterygians.—The fish-like mouth appears with the group of Crossopterygians, fishes which still retain the old-fashioned type of pectoral and ventral fin, the archipterygium. In the archaic tail, enameled scales, and cartilaginous skeleton the Crossopterygian shows its affinity with its Dipnoan ancestry. Thus these fishes unite in themselves traits of the shark, lung-fish, and Ganoid. The few living Crossopterygians, Polypterus and Erpetoichthys, are not very different from those which prevailed in Devonian times. The larvæ possess external gills with firm base and fringe-like rays, suggesting a resemblance to the pectoral fin itself, which develops from the shoulder-girdle just below it and would seem to give some force to Kerr's contention that the archipterygium is only a modified external gill. In Polypterus the archipterygium has become short and fan-shaped, its axis made of two diverging bones with flat cartilage between. From this type it is thought that the arm of the higher forms has been developed. The bony basis may be the humerus, from which diverge radius and ulna, the carpal bones being formed of the intervening cartilage.
Fig. 260.—An extinct Crossopterygian, Holoptychius giganteus Agassiz (1835). (After Agassiz, per Zittel.)
The Actinopteri.—From the Crossopterygians springs the main branch of the true fishes, known collectively as Actinopteri, or ray-fins, those with ordinary rays on the paired fins instead of the jointed archipterygium. The transitional series of primitive Actinopteri are usually known as Ganoids. The Ganoid differs from the Crossopterygian in having the basal elements of the paired fins small and concealed within the flesh. But other associated characters of the Crossopterygii and Dipnoans are preserved in most of the species. Among these are the mailed head and body, the heterocercal tail, the cellular air-bladder, the presence of valves in the arterial bulb, the presence of a spiral valve in the intestine and of a chiasma in the optic nerves. All these characters are found in the earlier types so far as is known, and all are more or less completely lost or altered in the teleosts or bony fishes. Among these early types is every variety of form, some of them being almost as long as deep, others arrow-shaped, and every intermediate form being represented. An offshoot from this line is the bowfin (Amia calva), among the Ganoids the closest living ally of the bony fishes, showing distinct affinities with the great group to which the herring and salmon belong. Near relatives of the bowfin flourished in the Mesozoic, among them some with a forked tail, and some with a very long one. From Ganoids of this type the vast majority of recent fishes may be descended.
Fig. 261.—An ancient Ganoid fish, Platysomus gibbosus Blainville. Family Platysomidæ. (After Woodward.)
Fig. 262.—A living Ganoid fish, the Short-nosed Gar, Lepisosteus platystomus Rafinesque. Lake Erie.
Another branch of Ganoids, divergent from both garfish and bowfin and not recently from the same primitive stock, included the sturgeons (Acipenser, Scaphirhynchus, Kessleria) and the paddle-fishes (Polyodon and Psephurus). All these are regarded by Woodward as degenerate descendants of the earliest Ganoids, Palæoniscidæ, of Devonian and Carboniferous time.
Fig. 263.—A primitive Ganoid fish, Palæoniscum macropomum (Agassiz), restored. Permian. Family Potaconiscidæ. (After Traquair.)
Fig. 264.—A fossil Herring, Diplomystus humilis Leidy. (From a specimen obtained at Green River, Wyo.) The scutes along the back lost in the specimen. Family Clupeidæ.
The Bony Fishes.—All the remaining fishes have ossified instead of cartilaginous skeletons. The dipnoan and ganoid traits one by one are more or less completely lost. Through these the main line of fish development continues and the various groups are known collectively as bony fishes or teleosts.
Fig. 265.—A primitive Herring-like fish, Holcolepis lewesiensis Mantell, restored. Family Elopidæ. English Chalk. (After Woodward.)
Fig. 266.—Ten-pounder, Elops saurus L. An ally of the earliest bony fishes. Virginia.
The earliest of the true bony fishes or teleosts appear in Mesozoic times, the most primitive forms being soft-rayed fishes with the vertebræ all similar in form, allied more or less remotely to the herring of to-day, but connected in an almost unbroken series with the earliest ganoid forms. In these and other soft-rayed fishes the pelvis still retains its posterior insertion, the ventral fins being said to be abdominal. The next great stage in evolution brings the pelvis forward, attaching it to the shoulder-girdle so that the ventral fins are now thoracic as in the perch and bass. If brought to a point in front of the pectoral fins, a feature of specialized degradation, they become jugular as in the codfish. In the abdominal fishes the air-bladder still retains its rudimentary duct joining it to the œsophagus.
From the abdominal forms allied to the herring, the huge array of modern fishes, typified by the perch, the bass, the mackerel, the wrasse, the globefish, the sculpin, the sea-horse, and the cod descended in many diverging lines. The earliest of the spine-rayed fishes with thoracic fins belong to the type of Berycidæ, a group characterized by rough scales, the retention of a primitive bone between the eyes, and the retention of the primitive larger number of ventral rays. These appear in the Cretaceous or chalk deposits, and show various attributes of transition from the abdominal to the thoracic type of ventrals.
Fig. 267.—Cardinal-fish, a perch-like fish, Apogon semilineatus Schlegel. Misaki, Japan.
Fig. 268.—Summer Herring, Pomolobus æstivalis (Mitchill). Potomac River. Family Clupeidæ.
Another line of descent apparently distinct from that of the herring and salmon extends through the characins to the loach, carps, catfishes, and electric eel. The fishes of this series have the anterior vertebræ coossified and modified in connection with the hearing organ, a structure not appearing elsewhere among fishes. This group includes the majority of fresh-water fishes. Still another great group, the eels, have lost the ventral fins and the bones of the head have suffered much degradation.
Fig. 269.—Fish with jugular ventral fins, Bassozetus catena Goode & Bean. Family Brotulidæ. Gulf Stream.
Fig. 270.—A specialized bony fish, Trachicephalus uranoscopus. Family Scorpænidæ. From Swatow, China.
The most highly developed fishes, all things considered, are doubtless the allies of the perch, bass, and sculpin. These fishes have lost the air-duct and on the whole they show the greatest development of the greatest number of structures. In these groups their traits one after another are carried to an extreme and these stages of extreme specialization give way one after another to phases of degeneration. The specialization of one organ usually involves degeneration of some other. Extreme specialization of any organ tends to render it useless under other conditions and may be one step toward its final degradation.
Fig. 271.—An African Catfish, Chlarias breviceps Boulenger. Congo River. Family Chlariidæ. (After Boulenger.)
Fig. 272.—Silverfin, Notropis whipplii (Girard). White River, Indiana. Family Cyprinidæ.
We have thus seen, in hasty review, that the fish-like vertebrates spring from an unknown and possibly worm-like stock, that from this stock, before it became vertebrate, degenerate branches have fallen off, represented to-day by the Tunicates and Enteropneustans. We have seen that the primitive vertebrate was headless and limbless and without hard parts. The lancelet remains as a possible direct offshoot from it; the cyclostome with brain and skull is a possible derivative from archaic lancelets. The earliest fishes leaving traces in the rocks were mailed ostracophores. From an unknown but possibly lamprey-like stock sprang the sharks and chimæras. The sharks developed into rays in one right line and into the highest sharks along another, while by a side branch through lost stages the primitive sharks passed into Crossopterygians, into Dipnoans, or lung-fishes, and perhaps into Ostracophores. All these types and others abound in the Devonian Age and the early records were lost in the Silurian. From the Crossopterygians or their ancestors or descendants by the specialization of the lung and limbs, the land animals, at first amphibians, after these reptiles, birds, and mammals, arose.
Fig. 273.—Moray, Gymnothorax moringa Bloch. Family Murænidæ Tortugas.
Fig. 274.—Amber-fish, Seriola lalandi (Cuv. & Val.). Family Carangidæ. Woods Hole.
In the sea, by a line still more direct, through the gradual emphasis of fish-like characters, we find developed the Crossopterygians with archaic limbs and after these the Ganoids with fish-like limbs but otherwise archaic; then the soft-rayed and finally the spiny-rayed bony fishes, herring, mackerel, perch, which culminate in specialized and often degraded types, as the anglers, globefishes, parrot-fishes, and flying gurnards; and from each of the ultimate lines of descent radiate infinite branches till the sea and rivers are filled, and almost every body of water has fishes fitted to its environment.
Geological Distribution of the Families of Elasmobranchs.
| Pliocene | Miocene | Eocene | Cretaceous | Jurassic | Triassic | Permian | Coal Measures | Sub-Carbonifero | Devonian | Silurian | Ordovician | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Cladoselachidæ | █ | █ | ||||||||||
| Acanthodii | █ | █ | █ | █ | ||||||||
| Pleuracanthidæ | █ | █ | █ | █ | ||||||||
| Cladodontidæ | █ | █ | █ | █ | ||||||||
| Petalodontidæ | █ | █ | █ | |||||||||
| Psammodontidæ | █ | █ | █ | |||||||||
| Cochliodontidæ | █ | █ | █ | |||||||||
| Orodontidæ | █ | █ | ||||||||||
| Heterodontidæ | █ | █ | █ | █ | █ | █ | █ | |||||
| Tamiobatidæ | █ | |||||||||||
| Hexanchidæ | █ | █ | █ | █ | █ | |||||||
| Lamnidæ | █ | █ | █ | █ | █ | |||||||
| Mitsukurinidæ | █ | █ | █ | █ | ||||||||
| Odontaspidæ | █ | █ | █ | █ | ||||||||
| Scyliorhinidæ | █ | █ | █ | █ | █ | |||||||
| Carchariidæ | █ | █ | █ | █ | ||||||||
| Squalidæ | █ | █ | █ | █ | ||||||||
| Dalatiidæ | █ | █ | █ | |||||||||
| Squatinidæ | █ | █ | █ | █ | █ | |||||||
| Rhinobatidæ | █ | █ | █ | █ | █ | |||||||
| Pristididæ | █ | █ | █ | █ | ||||||||
| Rajidæ | █ | █ | █ | █ | ||||||||
| Narcobatidæ | █ | █ | █ | |||||||||
| Dasyatidæ | █ | █ | █ | █ | ||||||||
| Myliobatidæ | █ | █ | █ | █ | █ | |||||||
| Ptychodontidæ | █ | |||||||||||
| Chimæridæ | █ | █ | █ | █ | █ |
[CHAPTER XXV]
THE PROTOCHORDATA
The Chordate Animals.—Referring to our metaphor of the tree with its twigs as used in the chapter on classification we find the fishes with the higher vertebrates as parts of a great branch from which the lower twigs have mostly perished. This great branch, phylum, or line of descent is known in zoology as Chordata, and the organisms associated with it or composing it are chordate animals.
The chordate animals are those which at some stage of life possess a notochord or primitive dorsal cartilage which divides the interior of the body into two cavities. The dorsal cavity contains the great nerve centers or spinal cord; the ventral cavity contains the heart and alimentary canal. In all other animals which possess a body cavity, there is no division by a notochord, and the ganglia of the nervous system if existing are placed on the ventral side or in a ring about the mouth.
The Protochordates.—Modern researches have shown that besides the ordinary back-boned animals certain other creatures easily to be mistaken for mollusks or worms and being chordate in structure must be regarded as offshoots from the vertebrate branch. These are degenerate allies, as is shown by the fact that their vertebrate traits are shown in their early or larval development and scarcely at all in their adult condition. As Dr. John Sterling Kingsley has well said: "Many of the species start in life with the promise of reaching a point high in the scale, but after a while they turn around and, as one might say, pursue a downward course, which results in an adult which displays but few resemblances to the other vertebrates." In the Tunicates or Ascidians (sea-squirts, sea-pears, and salpas), which constitute the class known as Tunicata or Urochordata, there is no brain, the notochord is confined to the tail and is usually present only in the larval stage of the animal when it has the form of a tadpole. In later life the animal usually becomes quiescent, attached to some hard object, fixed or floating. It loses its form and has the appearance of a hollow, leathery sac, the body organs being developed in a tough tunic. There are numerous families of Tunicates and the species are found in nearly all seas. They suggest no resemblance to fishes and look like tough clams without shells. The internal cavity being usually filled with water it is squirted out through the two apertures when the animal is handled. The class Enteropneusta (Adelochorda, or Hemichordata), includes the rather rare worm-like forms related to Balanoglossus. Bateson has shown that these animals possess a notochord which is developed in the anterior part of the body. They have no fins and before the mouth is a long proboscis. Gill-slits are found in the larval tunicate. In Balanoglossus these persist through life as in the fishes.
The remaining chordate forms constitute the vertebrates proper, not worm-like nor mollusk-like, the notochord not disappearing with age, except as it gives way, by specialized segmentation to the complex structures of the vertebral column. These vertebrates, which are permanently aquatic, are known in a popular sense as fishes. The fish, in the broad sense, is a back-boned animal which retains the homologue of the back-bone throughout life, which does not develop jointed limbs, its locomotive members, if present, being developed as fins, and which breathes through life the air contained in water by means of gills. This definition excludes the Tunicates and Enteropneusta on the one hand and the Amphibia or Batrachia with the reptiles, birds, and mammals on the other. The Amphibia are much more closely related to certain fishes than the classes of fishes are to each other. Still for purposes of systematic study, the frogs and salamanders are left out of the domain of ichthyology, while the Tunicata and the Enteropneusta might well be included in it.
The known branchiferous or gill-bearing chordates living and extinct may be first divided into eight classes—the Enteropneusta, the Tunicata, the Leptocardii, or lancelets, the Cyclostomi, or lampreys, the Elasmobranchii, or sharks, the Ostracophori the Arthrodira, and the Teleostomi, or true fishes. The first two groups, being very primitive and in no respect fish-like in appearance, are sometimes grouped together as Protochordata, the others with the higher Chordates constituting the Vertebrata.
Other Terms used in Classification.—The Leptocardii are sometimes called Acraniata (without skull), as distinguished from the higher groups, Craniota, in which the skull is developed. The Leptocardii, Cyclostomi, and Ostracophori are sometimes called Agnatha (without jaws) in contradistinction to the Gnathostomi (jaw mouths), which include the sharks and true fishes with the higher vertebrates. The sharks and Teleostomes are sometimes brought together as Pisces, or fishes, as distinguished from other groups not true fishes. To the sharks and true fishes the collective name of Lyrifera has been given, these fishes having the harp-shaped shoulder-girdle, its parts united below. The Ostracophores and Arthrodires agreeing in the bony coat of mail, and both groups now extinct and both of uncertain relationship, have been often united under the name of Placoderms, and these and many other fishes have been again erroneously confounded with the Ganoids. Again, the Teleostomi have been frequently divided into three classes—Crossopterygii, Dipneusti or Dipnoi, and Actinopterygii. The latter may be again divided into Ganoidei and Teleostei and all sorts of ranks have been assigned to each of these groups. For our purposes a division into eight classes is most convenient, and lowest among these we may place the Enteropneusta.
The Enteropneusta.—Most simple, most worm-like, and perhaps most primitive of all the Chordates is the group of worm-shaped forms, forming the class of Enteropneusta. The class of Enteropneusta, also called Adelochorda or Hemichordata, as here recognized, consists of a group of small marine animals allied to the genus Balanoglossus, or acorn-tongues (βάλανος, acorn; γλώσσα, tongue). These are worm-like creatures with fragile bodies buried in the sand or mud, or living under rocks of the seashore and in shallow waters, where they lie coiled in a spiral, with little or no motion. From the surface of the body a mucous substance is secreted, holding together particles by which are formed tubes of sand. The animal has a peculiar odor like that of iodoform. At the front is a long muscular proboscis, very sensitive, capable of great extension and contraction, largely used in burrowing in the ground, and of a brilliant orange color in life. Behind this is a collar which overlaps the small neck and conceals the small mouth at the base of the proboscis. The gill-slits behind the collar are also more or less concealed by it.
The body, which is worm-like, extends often to the length of two or three feet. The gill-slits in the adult are arranged in regular pairs, there being upwards of fifty in number much like the gill-slits of the lancelet. As the animal grows older the slits become less conspicuous, their openings being reduced to small slit-like pores.
In the interior of the proboscis is a rod-like structure which arises as an outgrowth of the alimentary canal above the mouth. In development and structure this rod so resembles the notochord of the lancelet that it is regarded as a true notochord, though found in the anterior region only. From the presence of gill-slits and notochord and from the development and structure of the central nervous system Balanoglossus was recognized by William Bateson, who studied an American species, Dolichoglossus kowalevskii, at Hampton Roads in Virginia in 1885, and at Beaufort in North Carolina, as a member of the Chordate series. Unlike the Tunicates it represents a primitively simple, not a degenerate, type. It seems to possess real affinities with the worms, or possibly, as some have thought, with the sea-urchins.
Fig. 275.—"Tornaria" Larva of Glossobalanus minutus. (After Minot.)
A peculiar little creature, known as Tornaria, was once considered to be the larva of a starfish. It is minute and transparent, floating on the surface of the sea. It has no visible resemblance to the adult Balanoglossus, but it has been reared in aquaria and shown to pass into the latter or into the related genus Glossobalanus. No such metamorphosis was found by Bateson in the more primitive genus Dolichoglossus, studied by him. This adult animal may be, indeed, a worm as it appears, but the presence of gill-slits, the existence of a rudimentary notochord, and the character of the central nervous system are distinctly fish-like and therefore vertebrate characters. With the Chordates, and not with the worms, this class, Enteropneusta (ἔντερον, intestine; πνεῖν, to breathe), must be placed if its characters have been rightly interpreted. It is possibly a descendant of the primitive creatures which marked the transition from the archaic worms, or possibly archaic Echinoderms, to the archaic Chordate type.
Fig. 276.—Glossobalanus minutus, one of the higher Enteropneustans. (After Minot.)
It is perhaps not absolutely certain that the notochord of Balanoglossus and its allies is a true homologue of the notochord of the lancelet. There may be doubt even of the homologies of the gill-slits themselves. But the balance of evidence seems to throw Balanoglossus on the fish side of the dividing line which separates the lower Chordates from the worms.
It may be noticed that Hubrecht regards the proboscis of various marine Nemertine worms as a real homologue of the notochord, and other writers have traced with more or less success other apparent or possible homologies between the Chordate and the Annelid series.
Classification of Enteropneusta.—Until recently the Enteropneusta have been usually placed in a single family or even in a single genus. The recent researches of Professor J. W. Spengel of Giessen and of Professor William Emerson Ritter of the University of California, have shown clearly that the group is much larger than had been generally supposed, with numerous species in all the warm seas. In Spengel's recent paper, "Die Benennung der Enteropneusten-Gattungen," three families are recognized with nine genera and numerous species. At least seven species are now known from the Pacific Coast of North America.
Family Harrimaniidæ.—In Harrimania maculosa, lately described by Dr. Ritter from Alaska, the eggs are large, with much food yolk, and the process of development is probably, without Tornaria stage. A second species of Harrimania (H. kupferi) is now recognized from Norway and Greenland. This genus is the simplest in structure among all the Enteropneustans and may be regarded as the lowest of known Chordates, the most worm-like of back-boned animals.
Fig. 277.—Harrimania maculosa (Ritter), the lowest of chordate animals. An Enteropneustan from Alaska. (After Ritter.)
In Dolichoglossus kowalevskii the species studied by Bateson on the Virginia coast, the same simplicity of development occurs. This genus, with a third, Stereobalanus (canadensis), constitutes in Spengel's system the family of Harrimaniidæ.
Balanoglossidæ.—The family Glandicepitidæ contains the genera Glandiceps, Spengelia, and Schizocardium. In the Balanoglossidæ (Ptychoderidæ of Spengel) the eggs are very small and numerous, with little food yolk. The species in this family pass through the Tornaria stage above described, a condition strikingly like that of the larval starfish. This fact has given rise to the suggestion that the Enteropneusta have a real affinity with the Echinoderms.
The Balanoglossidæ include the genera Glossobalanus, Balanoglossus, and Ptychodera, the latter the oldest known member of the group, its type, Ptychodera flava, having been described by Eschscholtz from the Pacific Coast in 1825, while Balanoglossus clavigerus was found by Della Chiaje in 1829.
Low Organization of Harrimaniidæ.—Apparently the Harrimaniidæ, with simpler structure, more extensive notochord, and direct development, should be placed at the bottom as the most primitive of the Enteropneustan series. Dr. Willey, however, regards its characters as due to degeneration, and considers the more elaborate Balanoglossidæ as nearest the primitive type. The case in this view would have something in common with that of the Larvacea, which seems to be the primitive Tunicates, but which may have been produced by the degeneration of more complex forms.
[CHAPTER XXVI]
THE TUNICATES, OR ASCIDIANS
Structure of Tunicates.—One of the most singular groups of animals is that known as Ascidians, or Tunicates. It is one of the most clearly marked yet most heterogeneous of all the classes of animals, and in no other are the phenomena of degeneration so clearly shown.
Among them is a great variety of form and habit. Some lie buried in sand; some fasten themselves to rocks; some are imbedded in great colonies in a gelatinous matrix produced from their own bodies, and some float freely in long chains in the open sea. All agree in changing very early in their development from a free-swimming or fish-like condition to one of quiescence, remaining at rest or drifting with the current.
Says Dr. John Sterling Kingsley: "Many of the species start in life with the promise of reaching a point high in the scale, but after a while they turn around and, as one might say, pursue a downward course which results in an adult which displays but few resemblances to the other vertebrates. Indeed, so different do they seem that the fact that they belong here was not suspected until about thirty-five years ago. Before that time, ever since the days of Cuvier, they were almost universally regarded as mollusks, and many facts were adduced to show that they belonged near the acephals (clams, oysters, etc.). In the later years when the facts of development began to be known, this association was looked on with suspicion, and by some they were placed for a short time among the worms. Any one who has watched the phases of their development cannot help believing that they belong here, the lowest of the vertebrate series."
The following account of the structure and development of the Tunicate is taken, with considerable modification and condensation, from Professor Kingsley's chapter on the group in the Riverside Natural History. For the changes suggested I am indebted to the kindness of Professor William Emerson Ritter:
The Tunicates derive their name from the fact that the whole body is invested with a tough envelope or "tunic." This tunic or test may be either gelatinous, cartilaginous, or leathery. In some forms it is perfectly transparent, in others it is translucent, allowing enough light to pass to show the colors of the viscera, while in still others it is opaque and variously colored. The tunic is everywhere only loosely attached to the body proper, except in the region of the two openings now to be mentioned. One of these openings occupies a more or less central position, while the other is usually at one side, or it may even be placed at the opposite end of the body. On placing one of the Ascidians in a glass dish and sprinkling a little carmine or indigo in the water, we can study some of the functions of the animal. As soon as the disturbance is over, the animals will open the two apertures referred to, when it will be seen that each is surrounded with blunt lobes, the number of which varies with the species. As soon as they are opened a stream of water will be seen to rush into the central opening, carrying with it the carmine, and a moment later a reddish cloud will be ejected from the other aperture. From this we learn that the water passes through the body. Why it does so is to be our next inquiry. On cutting the animal open we find that the water, after passing through the first-mentioned opening (which may be called the mouth) enters a spacious chamber, the walls of which are made up of fine meshes, the whole appearing like lattice-work. Taking out a bit of this network and examining it under the microscope, we find that the edges of the meshes are armed with strong cilia, which are in constant motion, forcing the water through the holes. Of course, the supply has to be made good, and hence more water flows in through the mouth. This large cavity is known as the branchial or pharyngeal chamber. It is, according to Professor Ritter, "as we know from the embryology of the animal, the greatly enlarged anterior end of the digestive tract; and as the holes, or stigmata, as they are technically called, are perforations of the wall for the passage of water for purposes of respiration, they are both morphologically and physiologically comparable with the gill openings of fishes." There can be no doubt, therefore, that the pharyngeal sac of Ascidians is homologous with the pharynx of fishes.
Surrounding the mouth, or branchial orifice, just at its entrance into the branchial chamber is a circle of tentacles. These are simple in some genera, but elaborately branched in others.
In close connection with the cerebral ganglion, which is situated between the two siphons, there is a large gland with a short trumpet-shaped duct opening into the branchial sac a little distance behind the mouth. The orifice of the duct is just within a ring consisting of a ciliated groove that extends around the mouth outside the circle of branchial tentacles. On the opposite side of the mouth from the gland the ciliated groove joins another groove which is both ciliated and glandular, and which runs backward along the upper floor of the pharyngeal sac to its posterior extremity. This organ, called the endostyle, is concerned in the transportation of the animal's food through the pharyngeal sac to the opening of the œsophagus. Comparative embryology makes it almost certain that the subneural gland with its duct, described above, is homologous with the hypophesis cerebri of true vertebrates, and that the endostyle is homologous with the thyroid glands of vertebrates.
The water after passing through the branchial network is received into narrow passages and conducted to a larger cavity—the cloacal or atrial chamber. The general relations can he seen from our diagram, illustrating a vertical and horizontal section. From the atrial chamber the water flows out into the external world.
Now we can readily see how in the older works naturalists were misled as to the affinities of the Tunicates. They regarded the tunic as the equivalent of the mantle of the mollusks, while the incurrent and excurrent openings corresponded to the siphons. In one genus, Rhodosoma, the resemblance was even stronger, for there the tunic is in two parts, united by a hinge line, and closed by an adductor muscle. How and why these views were totally erroneous will be seen when we come to consider the development of these animals.
At the bottom of the pharnygeal sac is the narrow œsophagus surrounded with cilia, which force a current down into the digestive tract. The branchial meshes serve as a strainer for the water, and the larger particles which it contains fall down until they are within reach of the current going down the œsophagus. After passing through the throat, they come to the stomach, where digestion takes place, and then the ejectamenta are carried out through the intestine and poured into the bottom of the atrial cavity.
The heart lies on the ventral side of the stomach and is surrounded by a well-developed pericardium. The most remarkable fact connected with the circulation is that the heart, after beating a short time, forcing the blood through the vessels, will suddenly stop for a moment and then resume its beats; but, strange to say, after the stoppage the direction of the circulation is reversed, the blood taking an exactly opposite course from that formerly pursued. This most exceptional condition was first seen in the transparent Salpa, but it may be witnessed in the young of most genera. We have already referred to the branchial chamber. The walls of this chamber, besides acting as a strainer, are also respiratory organs. The meshes of which they are composed are in reality tubes through which the blood circulates and thus is brought in contact with a constantly renewed supply of fresh water.
The central nervous system in the adults of all except the Larvacea is reduced to a single ganglion placed near the mouth thus indicating the dorsal side. In forms like Cynthia it holds the same relative position with regard to the mouth, but by the doubling of the body (to be explained further on) it is also brought near the atrial aperture, where it is shown in our first diagram.
Development of Tunicates.—The sexes are combined in the same individual, though usually the products ripen at different times. As a rule, the earlier stages of the embryo are passed inside the cloacal chamber, though in some the development occurs outside the body. As a type of the development we will consider that of one of the solitary forms, leaving the many curious modifications to be noticed in connection with the species in which they occur. This will be best, since these forms show the relationship to the other vertebrates in the clearest manner.
Fig. 278.—Development of the larval Tunicate to the fixed condition. (From Seeliger, per Parker & Haswell.) a, larva; b, intermediate stage; c, adult.
The egg undergoes a total segmentation and a regular gastrulation. Soon a tail appears, and under the microscope the young embryo, which now begins its free life, appears much like the tadpole of the frog. It has a large oval body and a long tail which lashes about, forcing the animal forward with a wriggling motion. Nor is the resemblance superficial; it pervades every part of the structure, as may be seen from the adjacent diagram. The mouth is nearly terminal and communicates with a gill-chamber provided with gill-clefts. At the posterior end of the gill-chamber begins the alimentary tract, which pursues a convoluted course to the vent. In the tail, but not extending to any distance into the body, is an axial cylinder, the notochord, which here, as in all other vertebrates, arises from the hypoblast; and above it is the spinal cord (epiblastic in origin), which extends forward to the brain, above the gill-chamber. Besides, the animal is provided with organs of sight and hearing, which, however, are of peculiar construction and can hardly be homologized with the corresponding organs in vertebrates. So far the correspondence between the two types is very close, and if we knew nothing about the later stages, one would without doubt predict that the adult tunicate would reach a high point in the scale of vertebrates. These high expectations are never fulfilled; the animal, on the contrary, pursues a retrograde course, resulting in an adult whose relationship to the true vertebrates never would have been suspected had its embryology remained unknown.
Fig. 279.—Anatomy of Tunicate. (After Herdman, per Parker & Haswell.)
After the stage described this retrograde movement begins. From various parts of the body lobes grow out, armed on their extremities with sucking-disks. These soon come in contact with some subaquatic object and adhere to it. Then the notochord breaks down, the spinal cord is absorbed, the tail follows suit, the intestine twists around, and the cloaca is formed, the result being much like the diagram near the head of this section. In forms like Appendicularia, little degeneration takes place, so far as is known, the tail, with its notochord and neural chord, persisting through life.
Reproduction of Tunicates.—As to the reproduction of the Tunicates, Dr. Ritter writes: "In addition to the sexual method of reproduction, many tunicates reproduce asexually by budding. The capacity for bud reproduction appears to have been acquired by certain simple Ascidians in connection with, probably as a result of, their having given up the free-swimming life and become attached and consequently degenerate.
"Instructive as the embryonic development of the creatures is from the standpoint of evolution, the bud method of development is scarcely less so from the same point of view. The development of the adult zooid from the simple bud has been conclusively shown to be by a process in many respects fundamentally unlike that by which the individual is developed from the egg. We have then in these animals a case in which practically the same results are reached by developmental processes that are, according to prevailing conceptions of animal organizations, fundamentally different. This fact has hardly a parallel in the animal kingdom."
Habits of Tunicates.—The Tunicates are all marine, some floating or swimming freely, some attached to rocks or wharves, others buried in the sand. They feed on minute organisms, plants, or animals, occasional rare forms being found in their stomachs. Some of them possess a single median eye or eye-like structure which may not do more than recognize the presence of light. No fossil Tunicates are known, as they possess no hard parts, although certain Ostracoderms have been suspected, though on very uncertain grounds, to be mailed Tunicates, rather than mailed lampreys. It is not likely that this hypothesis has any sound foundation. The group is divided by Herdman and most other recent authorities into three orders, viz., the Larvacea, the Ascidiacea, and the Thaliacea.
Larvacea.—In the most primitive order the animals are minute and free-swimming, never passing beyond the tadpole stage. The notochord and the nervous chord persist through life, the latter with ganglionic segmentations at regular intervals. The species mostly float in the open sea, and some of them form from their own secretions a transparent gelatinous envelope called a "house." This has two apertures and a long chamber "in which the tail has room to vibrate."
The order consists of a single small family, Appendiculariidæ. The lowest type is known as Kowalevskia, a minute creature without heart or intestine found floating in the Mediterranean. It is in many respects the simplest in structure among Chordate animals. Oikopleura (Fig. 288) is another genus of this group.
Ascidiacea.—In the Ascidiacea the adult is usually attached to some object, and the two apertures are placed near each other by the obliteration of the caudal area. The form has been compared to a "leathern bottle with two spouts."
Fig. 280.—Ascidia adhærens Ritter. Glacier Bay, Alaska. (After Ritter.)
The suborder Ascidiæ simplices includes the solitary Ascidians or "sea-squirts," common on our shores, as well as the social forms in which an individual is surrounded by its buds. The common name arises from the fact that when touched they contract, squirting water from both apertures. The Ascidiidæ comprise the most familiar solitary forms, some of them the largest of the Tunicates and represented on most coasts. In the Molgulidæ and most Ascidiæ compositæ the young hatch out in the cloaca, from which "these tadpoles swim out as yellow atoms," while in a new genus, Euherdmania, described by Ritter, from the coast of California, the embryos are retained through their whole larval stage in the oviduct of the parent. They form, according to Kingsley, adhesive processes on the body, but those of Molgula cannot use them in becoming attached to rocks, since they are entirely inclosed in a peculiar envelope. This envelope is after a while very adhesive, and if the little tadpole happens to touch any part of himself to a stone or shell he is fastened for life. Thus "I have frequently seen them adhere by the tail, while the anterior part was making the most violent struggles to escape. Soon, however, they settle down contentedly, absorb the tail, and in a few weeks assume the adult structure."
In the family Cynthiidæ the brightly-colored red and yellow species of Cynthia are known as sea-peaches by the fishermen. The sea-pears, Boltenia, are fastened to long stalks. These have a leathery and wrinkled tunic, to which algæ and hydroids freely attach themselves. Into the gill-cavity of these forms small fishes, blennies, gobies, and pearl-fishes often retreat for protection.
Fig. 281.—Styela yacutatensis (Ritter), a simple Ascidian. Family Molgulidæ. Yakutat Bay, Alaska. (After Ritter.)
The social Ascidians constitute the Clavellinidæ. They are similar to the Ascidiidæ in form, but each individual sends out a bud which forms a stern bearing another individual at the end. By this means large colonies may be formed.
The suborder, Ascidiæ compositæ, contains the compound Ascidians or colonies enveloped in a common gelatinous "test." These colonies are usually attached to rock or seaweed, and the individuals are frequently regularly and symmetrically arranged. The bodies are sometimes complex in form.
Fig. 282.—Styela greeleyi Ritter. Family Molgulidæ. Lukanin, Pribilof Islands. (After Ritter.)
Fig. 283.—Cynthia superba Ritter. A Tunicate from Puget Sound. Family Cynthiidæ. (After Ritter.)
In the Botryllidæ and Polystyelidæ the individuals are not segmented and in the former family are arranged in star-shaped groups about a common cloaca, into which the atrial siphons of the different individuals open. The group springs by budding from the tadpole, or larva, which has attached itself to some object. These forms are often brightly colored. Botryllus gouldi is a species very common along our North Atlantic coast, forming gray star-shaped masses sometimes an inch across on eel-grass (Zostera) and on flat-leaved seaweeds. Goodsiria dura, a representative of the Polystyelidæ, is one of the most common Ascidians on the California coast southward, where the brick-red masses incrusting on seaweeds of various kinds, and on other Ascidians, are frequently thrown ashore in great quantities during heavy storms.
Fig. 284.—Botryllus magnus Ritter. A compound Ascidian. Shumagin Islands, Alaska. (After Ritter.)
In Didemnidæ the body is more complex, of two parts, called the "thorax" and "abdomen." In Amarœcium, the "sea pork" of the fishermen, the body is in three parts and the individuals are very long. These sometimes form great masses a foot or more long, "colored like boiled salt pork, but more translucent." Other families of this type are the Distomidæ and the Polyclinidæ.
In the suborder Luciæ, including the family Pyrosomidæ, the colonies are thimble-shaped and hollow, the incurrent openings being on the outer surface of the thimble, the outgoing stream opening within. Pyrosoma is highly phosphorescent. In the tropical seas some colonies reach a length of two or three feet. It is said that a description of a colony was once written by a naturalist on a page illumined by the colony's own light. "Each of the individuals has a number of cells near the mouth the function of which is to produce the light."
Thaliacea.—In the order Thaliacea the Tunicates have the two orifices at opposite ends of the body. All are free-swimming and perfectly transparent. The principal family is that of Salpidæ. The gill-cavity in Salpa is much altered, the gills projecting into it dividing it into two chambers.
In these forms we have the phenomena of alternation of generations. A sexual female produces eggs, and from each hatches a tadpole larva which is without sex. This gives rise to buds, some at least of the individuals arising which in turn produce eggs.
Fig. 285.—Botryllus magnus Ritter. Part of colony. (After Ritter.)
In the family Salpidæ two kinds of individuals occur, the solitary salpa, or female, and the chain salpa, or bisexual males. The latter are united together in long bands, each individual forming a link in the chain held together by spurs extending from one to the next. From each solitary individual a long process or cord grows out, this dividing to form the chain. Each chain salpa produces male reproductive organs and each develops as well a single egg. The egg is developed within the body attached by a sort of placenta, while the spermatozoa are cast into the sea to fertilize other eggs. From each egg develops the solitary salpa and from her buds the chain of bisexual creatures. Dr. W. K. Brooks regards these as nursing males, the real source of the egg being perhaps the solitary female. Of this extraordinary arrangement the naturalist-poet Chamisso, who first described it, said: "A salpa mother is not like its daughter or its own mother, but resembles its sister, its granddaughter, and its grandmother." But it is misleading to apply such terms taken from the individualized human relationship to the singular communal system developed by these ultra-degenerate and strangely specialized Chordates.
Fig. 286.—Botryllus magnus Ritter, a single Zooid. Shumagin Islands, Alaska. (After Ritter.)
Fig. 287.—Aplidiopsis jordani Ritter, a compound Ascidian. Lukanin Beach, Pribilof Islands. (After Ritter.)
The Salpas abound in the warm seas, the chains often covering the water for miles. They are perfectly transparent, and the chains are often more than a foot in length. In Doliolum the body is barrel-shaped and the gills are less modified than in Salpa. The alternation of generations in this genus is still more complicated than in Salpa, for here we have not only a sexual and a non-sexual generation, the individuals of which differ from each other, but there is further a differentiation among the asexually produced individuals themselves; so that we have in all three instead of two sorts of animals in the complete life cycle. Besides the proliferating stolon situated on the ventral side, the bud-producing individual possesses a dorsal process larger than the stolon proper. The buds become completely severed from the true stolon at an early stage and actually crawl along the side of the parent up to the dorsal process, upon which they arrange themselves in three rows, two lateral and one median. The buds of the lateral rows become nutritive and respiratory zooids, while those of the median row, ultimately at least, give rise in turn to the egg-producing individuals.
Origin of Tunicates.—There can be little doubt that the Tunicata form an offshoot from the primitive Chordate stock, and the structure of their larva in connection with that of the lancelet throws a large light on the nature of their common parents. "We may conclude," says Dr. Arthur Willey, "that the proximate ancestor of the Vertebrates was a free-swimming animal intermediate in organization between an Ascidian tadpole and Amphioxus, possessing the dorsal mouth, hypophysis, and restricted notochord of the former and the myotomes, cœlomic epithelium, and straight alimentary canal of the latter. The ultimate or primordial ancestor of the Vertebrates would, on the contrary, be a worm-like animal whose organization was approximately on a level with that of the bilateral ancestors of the Echinoderms."
Fig. 288.—Adult Tunicate of the group Larvacea, Oikopleura. Family Appendiculariidæ. (After Fol, per Parker & Haswell.)
Degeneration of Tunicates.—There is no question, furthermore, Professor Ritter observes, "that most of the group has undergone great degeneration in its evolutionary course. Just what the starting-point was, however, is a matter on which there is considerable difference of opinion among authorities. According to one view, particularly championed by Professor W. K. Brooks, Appendicularia is very near the ancestral form. The ancestor was consequently a small, marine, free-swimming creature. From this ancestor the Ascidiacea were evolved largely through the influence of the attached habit of life, and the tadpole stage in their development is a recapitulation of the ancestral form, just as the tadpole stage in the frog's life is a repetition of the fish ancestry of the frog.
"According to the most common view Appendicularia is not an ancestral form at all, but is the tadpole stage of the Ascidiacea that has failed to undergo metamorphosis and has become sexually mature in the larval condition, as the larva of certain Amphibians and insects are known to never pass into the adult state but reproduce their kind sexually in the larval condition. By this view the tadpole of such Ascidian as Ciona, for example, represents more closely the common ancestor of the group than does any other form we know. This view is especially defended by Professor K. Heider and Dr. Arthur Willey."
[CHAPTER XXVII]
THE LEPTOCARDII, OR LANCELETS
The Lancelet.—The lancelet is a vertebrate reduced to its very lowest terms. The essential organs of vertebrate life are there, but each one in its simplest form unspecialized and with structure and function feebly differentiated. The skeleton consists of a cartilaginous notochord inclosed in a membranous sheath. There is no skull. No limbs, no conspicuous processes, and no vertebræ are present. The heart is simply a long contractile tube, hence the name Leptocardii (from λεπτός, slender; καρδία, heart). The blood is colorless. There is a hepatic portal circulation. There is no brain, the spinal cord tapering in front as behind. The water for respiration passes through very many gill-slits from the pharynx into the atrium, from which it is excluded through the atripore in front of the vent. A large chamber, called the atrium, extends almost the length of the body along the ventral and lateral regions. It communicates with the pharynx through the gill-slits and with the exterior through a small opening in front of the vent, the atripore. The atrium is not found in forms above the lancelets.
The reproductive organs consist of a series of pairs of segmentally arranged gonads. The excretory organs consist of a series of tubules in the region of the pharynx, connecting the body-cavity with the atrium. The mouth is a lengthwise slit without jaws, and on either side is a row of fringes. From this feature comes the name Cirrostomi, from cirrus, a fringe of hair, and στόμα, mouth. The body is lanceolate in form, sharp at either end. From this fact arises a third name, Amphioxus, from ἀμφί, both; ὀξύς, sharp. Dorsal and anal fins are developed as folds of the skin supported by very slender rays. There are no other fins. The alimentary canal is straight, and is differentiated into pharynx and intestine; the liver is a blind sac arising from the anterior end of the intestine. A pigment spot in the wall of the spinal cord has been interpreted as an eye. Above the snout is a supposed olfactory pit which some have thought to be connected with the pineal structure. The muscular impressions along the sides are very distinct and it is chiefly by means of the variation in numbers of these that the species can be distinguished. Thus in the common lancelet of Europe, Branchiostoma lanceolatum, the muscular bands are 35+14+12=61. In the common species of the Eastern coasts of America, Branchiostoma caribæum, these are 35+14+9=58, while in the California lancelet, Branchiostoma californiense, these are 44+16+9=69.
Habits of Lancelets.—Lancelets are slender translucent worm-like creatures, varying from half an inch (Asymmetron lucayanum) to four inches (Branchiostoma californiense) in length. They live buried in sand in shallow waters along the coasts of warm seas. One species, Amphioxides pelagicus, has been taken at the depth of 1000 fathoms, but whether at the bottom or floating near the surface is not known. The species are very tenacious of life and will endure considerable mutilation. Some of them are found on almost every coast in semi-tropical and tropical regions.
Species of Lancelets.—The Mediterranean species ranges northward to the south of England. Others are found as far north as Chesapeake Bay, San Diego, and Misaki in Japan, where is found a species called Branchiostoma belcheri. The sands at the mouth of San Diego Bay are noted as producing the largest of the species of lancelets, Branchiostoma californiense. From the Bahamas comes the smallest, the type of a distinct genus, Asymmetron lucayanum, distinguished among other things by a projecting tail. Other supposed genera are Amphioxides (pelagicus), dredged in the deep sea off Hawaii and supposed to be pelagic, the mouth without cirri; Epigonichthys (cultellus), from the East Indies, and Heteropleuron (bassanum), from Bass Straits, Australia. These little animals are of great interest to anatomists as giving the clue to the primitive structure of vertebrates. While possibly these have diverged widely from their actual common ancestry with the fishes, they must approach near to these in many ways. Their simplicity is largely primitive, not, as in the Tunicates, the result of subsequent degradation.
Fig. 289.—California Lancelet, Branchiostoma californiense Gill. (From San Diego.)
The lancelets, less than a dozen species in all, constitute a single family, Branchiostomidæ. The principal genus, Branchiostoma, is usually called Amphioxus by anatomists. But while the name Amphioxus, like lancelet, is convenient in vernacular use, it has no standing in systematic nomenclature. The name Branchiostoma was given to lancelets from Naples in 1834, by Costa, while that of Amphioxus, given to specimens from Cornwall, dates from Yarrell's work on the British fishes in 1836. The name Amphioxus may be pleasanter or shorter or more familiar or more correctly descriptive than Branchiostoma, but if so the fact cannot be considered in science as affecting the duty of priority.
The name Acraniata (without skull) is often used for the lower Chordates taken collectively, and it is sometimes applied to the lancelets alone. It refers to those chordate forms which have no skull nor brain, as distinguished from the Craniota, or forms with a distinct brain having a bony or cartilaginous capsule for its protection.
Origin of Lancelets.—It is doubtless true, as Dr. Willey suggests, that the Vertebrates became separated from their worm-like ancestry through "the concentration of the central nervous system along the dorsal side of the body and its conversion into a hollow tube." Besides this trait two others are common to all of them, the presence of the gill-slits and that of the notochord. The gill-slits may have served primarily to relieve the stomach of water, as in the lowest forms they enter directly into the body-cavity. The primitive function of the notochord is still far from clear, but its ultimate use of its structures in affording protection and in furnishing a fulcrum for the muscles and limbs is of the greatest importance in the processes of life.
Fig. 289a.—Gill-basket of Lamprey.
[CHAPTER XXVIII]
THE CYCLOSTOMES, OR LAMPREYS
The Lampreys.—Passing upward from the lancelets and setting aside the descending series of Tunicates, we have a long step indeed to the next class of fish-like vertebrates. During the period this great gap represents in time we have the development of brain, skull, heart, and other differentiated organs replacing the simple structures found in the lancelet.
The presence of brain without limbs and without coat-of-mail distinguishes the class of Cyclostomes, or lampreys (κυκλός, round; στόμα, mouth). This group is also known as Marsipobranchi (μαρσιπίον, pouch; βράγχος, gill); Dermopteri (δέρμα, skin; πτερόν, fin); and Myzontes (μυζάω, to suck). It includes the forms known as lampreys, slime-eels, and hagfishes.
Structure of the Lamprey.—Comparing a Cyclostome with a lancelet we may see many evidences of specialization in structure. The Cyclostome has a distinct head with a cranium formed of a continuous body of cartilage modified to contain a fish-like brain, a cartilaginous skeleton of which the cranium is evidently a differentiated part. The vertebræ are undeveloped, the notochord being surrounded by its membranes, without bony or cartilaginous segments. The gills have the form of fixed sacs, six to fourteen in number, on each side, arranged in a cartilaginous structure known as "branchial basket" (fig. 289a), the elements of which are not clearly homologous with the gill-arches of the true fishes. Fish-like eyes are developed on the sides of the head. There is a median nostril associated with a pituitary pouch, which pierces the skull floor. An ear-capsule is developed. The brain is composed of paired ganglia in general appearance resembling the brain of the true fish, but the detailed homology of its different parts offers considerable uncertainty. The heart is modified to form two pulsating cavities, auricle and ventricle. The folds of the dorsal and anal fins are distinct, supported by slender rays.
The mouth is a roundish disk, with rasping teeth over its surface and with sharper and stronger teeth on the tongue. The intestine is straight and simple. The kidney is represented by a highly primitive pronephros and no trace exists of an air-bladder or lung. The skin is smooth and naked, sometimes secreting an excessive quantity of slime.
From the true fishes the Cyclostomes differ in the total absence of limbs and of shoulder and pelvic girdles, as well as of jaws. It has been thought by some writers that the limbs were ancestrally present and lost through degeneration, as in the eels. Dr. Ayers, following Huxley, finds evidence of the ancestral existence of a lower jaw. The majority of observers, however, regard the absence of limbs and jaws in Cyclostomes as a primitive character, although numerous other features of the modern hagfish and lamprey may have resulted from degeneration. There is no clear evidence that the class of Cyclostomes, as now known to us, has any great antiquity, and its members may be all degenerate offshoots from types of greater complexity of structure.
Supposed Extinct Cyclostomes.—No species belonging to the class of Cyclostomes has been found fossil. We may reason theoretically that the earliest fish-like forms were acraniate or lancelet-like, and that lamprey-like forms would naturally follow these, but this view cannot be substantiated from the fossils. Lancelets have no hard parts whatever, and could probably leave no trace in any sedimentary deposit. The lampreys stand between lancelets and sharks. Their teeth and fins at least might occasionally be preserved in the rocks, but no structures certainly known to be such have yet been recognized. It is however reasonably certain that the modern lamprey and hagfish are descendants, doubtless degraded and otherwise modified from species which filled the gap between the earliest chordate animals and the jaw-bearing sharks.
Conodontes.—Certain structures found as fossils have been from time to time regarded as Cyclostomes, but in all such cases there is doubt as to the real nature of the fossil relic in question or as to the proper interpretation of its relationship.
Thus the Conodontes of the Cambrian, Silurian, and Devonian have been regarded as lingual teeth of extinct Cyclostomes. The Cycliæ of the Devonian have been considered as minute lampreys, although the vertebral segments are highly specialized, to a degree far beyond the condition seen in the lampreys of to-day. The Ostracophores have been regarded as monstrous lampreys in coat of mail, and the possibility of a lamprey origin even for Arthrodires has been suggested. The Cycliæ and Ostracophori were apparently without jaws or limbs, being in this regard like the Cyclostomes, but their ancestry and relationships are wholly problematical.
Fig. 290.—Polygnathus dubium Hinde. A Conodont from the New York Devonian. (After Hinde.)
The nature of the Conodontes is still uncertain. In form they resemble teeth, but their structure is different from that of the teeth of any fishes, agreeing with that of the teeth of annelid worms. Some have compared them to the armature of Trilobites. Some fifteen nominal genera are described by Pander in Russia, and by Hinde about Lake Erie and Lake Ontario. Some of these, as Drepaniodus, are simple, straight or curved grooved teeth or tooth-like structures; others, as Prioniodus, have numerous smaller teeth or denticles at the base of the larger one.
Orders of Cyclostomes.—The known Cyclostomes are naturally divided into two orders, the Hyperotreta, or hagfishes, and the Hyperoartia, or lampreys. These two orders are very distinct from each other. While the two groups agree in the general form of the body, they differ in almost every detail, and there is much pertinence in Lankester's suggestions that each should stand as a separate class. The ancestral forms of each, as well as the intervening types if such ever existed, are left unrecorded in the rocks.
The Hyperotreta, or Hagfishes.—The Hyperotreta (ὑπερῴα, palate; τρετός, perforate), or hagfishes, have the nostril highly developed, a tube-like cylinder with cartilaginous rings penetrating the palate. In these the eyes are little developed and the species are parasitic on other fishes. In Polistotrema stouti, the hagfish of the coast of California, is parasitic on large fishes, rockfishes, or flounders. It usually fastens itself at the throat or isthmus of its host and sometimes at the eyes. Thence it works very rapidly to the inside of the body. It there devours all the muscular part of the fish without breaking the skin or the peritoneum, leaving the fish a living hulk of head, skin, and bones. It is especially destructive to fishes taken in gill-nets. The voracity of the Chilean species Polistotrema dombeyi is equally remarkable. Dr. Federico T. Delfin finds that in seven hours a hagfish of this species will devour eighteen times its own weight of fish-flesh. The intestinal canal is a simple tube, through which most of the food passes undigested. The eggs are large, each in a yellowish horny case, at one end of which are barbed threads by which they cling together and to kelp or other objects. In the California hagfish, Polistotrema stouti, great numbers of these eggs have been found in the stomachs of the males.
Fig. 291.—California Hagfish, Polistotrema stouti Lockington.
Similar habits are possessed by all the species in the two families, Myxinidæ and Eptatretidæ. In the Myxinidæ the gill-openings are apparently single on each side, the six gills being internal and leading by six separate ducts to each of the six branchial sacs. The skin is excessively slimy, the extensible tongue is armed with two cone-like series of strong teeth. About the mouth are eight barbels.
Of Myxine, numerous species are known—Myxine glutinosa, in the north of Europe; Myxine limosa, of the West Atlantic; Myxine australis, and several others about Cape Horn, and Myxine garmani in Japan. All live in deep waters and none have been fully studied. It has been claimed that the hagfish is male when young, many individuals gradually changing to female, but this conclusion lacks verification and is doubtless without foundation.
In the Eptatretidæ the gill-openings, six to fourteen in number, are externally separate, each with its own branchial sac as in the lampreys.
The species of the genus Eptatretus (Bdellostoma, Heptatrema, and Homea, all later names for the same group) are found only in the Pacific, in California, Chile, Patagonia, South Africa, and Japan. In general appearance and habits these agree with the species of Myxine. The species with ten to fourteen gill-openings (dombeyi: stouti) are sometimes set off as a distinct genus (Polistotrema), but in other regards the species differ little, and frequent individual variations occur. Eptatretus burgeri is found in Japan and Eptatretus forsteri in Australia.
The Hyperoartia, or Lampreys.—In the order Hyperoartia, or lampreys, the single nostril is a blind sac which does not penetrate the palate. The seven gill-openings lead each to a separate sac, the skin is not especially covered with mucus, the eyes are well developed in the adult, and the mouth is a round disk armed with rasp-like teeth, the comb-like teeth on the tongue being less developed than in the hagfishes. The intestine in the lampreys has a spiral valve. The eggs are small and are usually laid in brooks away from the sea, and in most cases the adult lamprey dies after spawning. According to Thoreau, "it is thought by fishermen that they never return, but waste away and die, clinging to rocks and stumps of trees for an indefinite period, a tragic feature in the scenery of the river-bottoms worthy to be remembered with Shakespeare's description of the sea-floor." This account is not far from the truth, as recent studies have shown.
The lampreys of the northern regions constitute the family of Petromyzonidæ. The larger species (Petromyzon, Entosphenus) live in the sea, ascending rivers to spawn, and often becoming land-locked and reduced in size by living in rivers only. Such land-locked marine lampreys (Petromyzon marinus unicolor) breed in Cayuga Lake and other lakes in New York. The marine forms reach a length of three feet. Smaller lampreys of other genera six inches to eighteen inches in length remain all their lives in the rivers, ascending the little brooks in the spring, clinging to stones and clods of earth till their eggs are deposited. These are found throughout northern Europe, northern Asia, and the colder parts of North America, belonging to the genera Lampetra and Ichthyomyzon. Other and more aberrant genera from Chile and Australia are Geotria and Mordacia, the latter forming a distinct family, Mordaciidæ. In Geotria, a large and peculiar gular pouch is developed at the throat. In Macrophthalmia chilensis from Chile the eyes are large and conspicuous.
Food of Lampreys.—The lampreys feed on the blood and flesh of fishes. They attach themselves to the sides of the various species, rasp off the flesh with their teeth, sucking the blood till the fish weakens and dies. Preparations made by students of Professor Jacob Reighard in the University of Michigan show clearly that the lamprey stomach contains muscular tissue as well as the blood of fishes. The river species do a great deal of mischief, a fact which has been the subject of a valuable investigation by Professor H. A. Surface, who has also considered the methods available for their destruction. The flesh of the lamprey is wholesome, and the larger species, especially the great sea lamprey of the Atlantic, Petromyzon marinus, are valued as food. The small species, according to Prof. Gage, never feed on fishes.
Fig. 292.—Lamprey, Petromyzon marinus L. Woods Hole, Mass.
Metamorphosis of Lampreys.—All lampreys, so far as known, pass through a distinct metamorphosis. The young, known as the Ammocœtes form, are slender, eyeless, and with the mouth narrow and toothless. From Professor Surface's paper on "The Removal of Lampreys from the Interior Waters of New York" we have the following extracts (slightly condensed):
Fig. 293.—Petromyzon marinus unicolor (De Kay). Mouth of Lake Lamprey, Cayuga Lake. (After Gage.)
Fig. 294.—Lampetra wilderi Jordan & Evermann. Larval Brook Lamprey in its burrow in a glass filled with sand. (After Gage.)
Fig. 295.—Lampetra wilderi Jordan & Evermann. Mouth of Brook Lamprey. Cayuga Lake. (After Gage.)
"In the latter part of the fall the young lampreys, Petromyzon marinus unicolor, the variety land-locked in the lakes of Central New York, metamorphose and assume the form of the adult. They are now about six or eight inches long. The externally segmented condition of the body disappears. The eyes appear to grow out through the skin and become plainly visible and functional. The mouth is no longer filled with vertical membranous sheets to act as a sieve, but it contains nearly one hundred and fifty sharp and chitinous teeth, arranged in rows that are more or less concentric and at the same time presenting the appearance of circular radiation. These teeth are very strong, with sharp points, and in structure each has the appearance of a hollow cone of chitin placed over another cone or papilla. A little below the center of the mouth is the oral opening, which is circular and contains a flattened tongue which bears finer teeth of chitin set closely together and arranged in two interrupted (appearing as four) curved rows extending up and down from the ventral toward the dorsal side of the mouth. Around the mouth is a circle of soft membrane finally surrounded by a margin of fimbriæ or small fringe. This completes the apparatus with which the lamprey attaches itself to its victims, takes its food, carries stones, builds and tears down its nest, seizes its mate, holds itself in position in a strong current, and climbs over falls."
Mischief Done by Lampreys.—"The most common economic feature in the entire life history of these animals is their feeding habits in this (spawning) stage, their food now consisting wholly of the blood (and flesh) of fishes. A lamprey is able to strike its suctorial mouth against a fish, and in an instant becomes so firmly attached that it is very rarely indeed that the efforts of the fish will avail to rid itself of its persecutor. When a lamprey attaches itself to a person's hand in the aquarium, it can only be freed by lifting it from the water. As a rule it will drop the instant it is exposed to the open air, although often it will remain attached for some time even in the open air, or may attach itself to an object while out of water.
"Nearly all lampreys that are attached to fish when they are caught in nets will escape through the meshes of the nets, but some are occasionally brought ashore and may hang on to their victim with bulldog pertinacity.
"The fishes that are mostly attacked are of the soft-rayed species, having cycloid scales, the spiny-rayed species with ctenoid scales being most nearly immune from their attacks. We think there may be three reasons for this: 1st, the fishes of the latter group are generally more alert and more active than those of the former, and may be able more readily to dart away from such enemies; 2d, their scales are thicker and stronger and appear to be more firmly imbedded in the skin, consequently it is more difficult for the lampreys to hold on and cut through the heavier coat-of-mail to obtain the blood of the victim; 3d, since the fishes of the second group are wholly carnivorous and in fact almost exclusively fish-eating when adult, in every body of water they are more rare than those of the first group, which are more nearly omnivorous. According to the laws and requirements of nature the fishes of the first group must be more abundant, as they become the food for those of the second, and it is on account of their greater abundance that the lampreys' attacks on them are more observed.
"There is no doubt that the bullhead, or horned pout (Ameiurus nebulosus), is by far the greatest sufferer from lamprey attacks in Cayuga Lake. This may be due in part to the sluggish habits of the fish, which render it an easy victim, but it is more likely due to the fact that this fish has no scales and the lamprey has nothing to do but to pierce the thick skin and find its feast of blood ready for it. There is no doubt of the excellency of the bullhead as a food-fish and of its increasing favor with mankind. It is at present the most important food- and market-fish of the State (New York), being caught by bushels in the early part of June when preparing to spawn. As we have observed at times more than ninety per cent. of the catch attacked by lampreys, it can readily be seen how very serious are the attacks of this terrible parasite which is surely devastating our lakes and streams."
Migration or "Running" of Lampreys.—"After thus feeding to an unusual extent, their reproductive elements (gonads) become mature and their alimentary canals commence to atrophy. This duct finally becomes so occluded that from formerly being large enough to admit a lead-pencil of average size when forced through it, later not even liquids can pass through, and it becomes nearly a thread closely surrounded by the crowding reproductive organs. When these changes commence to ensue, the lampreys turn their heads against the current and set out on their long journeys to the sites that are favorable for spawning, which here may be from two to eight miles from the lake. In this migration they are true to their instincts and habits of laziness in being carried about, as they make use of any available object, such as a fish, boat, etc., that is going in their direction, fastening to it with their suctorial mouths and being borne along at their ease. During this season it is not infrequent that as the Cornell crews come in from practice and lift their shells from the water, they find lampreys clinging to the bottoms of the boats, sometimes as many as fifty at one time. They are likely to crowd up all streams flowing into the lake, inspecting the bed of the stream as they go. They do not stop until they reach favorable spawning sites, and if they find unsurmountable obstacles in their way, such as vertical falls or dams, they turn around and go down-stream until they find another, up which they go. This is proved every spring by the number of adult lampreys which are seen temporarily in Pall Creek and Cascadilla Creek. In each of these streams, about a mile from its mouth, there is a vertical fall over thirty feet in height which the lampreys cannot surmount, and in fact they have never been seen attempting to do so. After clinging with their mouths to the stones at the foot of the falls for a few days, they work their way down-stream, carefully inspecting all the bottom for suitable spawning sites. They do not spawn in these streams because there are too many rocks and no sand, but finally enter the only stream (the Cayuga Lake inlet) in which they find suitable and accessible spawning sites.
Fig. 296.—Kamchatka Lamprey, Lampetra camtschatica (Tilesius). Kamchatka.
"The three-toothed lampreys (Entosphenus tridentatus) of the West Coast climb low falls or rapids by a series of leaps, holding with their mouths to rest, then jumping and striking again and holding, thus leap by leap gaining the entire distance.
"The lampreys here have never been known to show any tendency or ability to climb, probably because there are no rapids or mere low falls in the streams up which they would run. In fact, as the inlet is the only stream entering Cayuga Lake in this region which presents suitable spawning conditions and no obstructions, it can be seen at once that all the lampreys must spawn in this stream and its tributaries.
Fig. 297.—Oregon Lamprey, Entosphenus tridentatus, ascending a brook. (Modified from a photograph by Dr. H. M. Smith. Published by Prof. H. A. Surface.) Willamette River, Oregon.
"In 'running' they move almost entirely at night, and if they do not reach a suitable spawning site by daylight, they will cling to roots or stones during the day and complete their journey the next night. This has been proven by the positive observation of individuals. Of the specimens that run up early in the season, about four-fifths are males. Thus the males do not exactly precede the females, because we have found the latter sex represented in the stream as early in the season as the former, but in the earlier part of the season the number of the males certainly greatly predominates. This proportion of males gradually decreases, until in the middle of the spawning season the sexes are about equally represented, and toward the latter part of the season the females continue to come until they in turn show the greater numbers. Thus it appears very evident in general that the reproductive instinct impels the most of the males to seek the spawning ground before the most of the females do. However, it should be said that neither the males nor the females show all of the entirely sexually mature features when they first run up-stream in the beginning of the season, but later they are perfectly mature and 'ripe' in every regard when they first appear in the stream. When they migrate, they stop at the site that seems to suit their fancy, many stopping near the lake, others pushing on four or five miles farther up-stream. We have noted, however, that later in the season the lower courses become more crowded, showing that the late comers do not attempt to push up-stream as far as those that came earlier. Also it thus follows, from what was just said about late-running females, that in the latter part of the season the lower spawning beds are especially crowded with females. In fact, during the early part of the month of June we have found, not more than half a mile above the lowest spawning bed, as many as five females on a spawning nest with but one male; and in that immediate vicinity many nests indeed were found at that time with two or three females and but one male.
"Having arrived at a shoal which seems to present suitable conditions for a spawning nest, the individual or pair commences at once to move stones with its mouth from the centre to the margin of an area one or two feet in diameter. When many stones are thus placed, especially at the upper edge, and they are cleaned quite free of sediment and algæ, both by being moved and by being fanned with the tail, and when the proper condition of sand is found in the bottom of the basin thus formed, it is ready to be used as a spawning bed or nest. A great many nests are commenced and deserted. This has been left as a mystery in publications on the subject, but we are well convinced that it is because the lampreys do not find the requisites or proper conditions of bottom (rocks, sand, etc., as given below) to supply all their needs and fulfill all conditions for ideal sites. This desertion of half-constructed nests is just what would be expected and anticipated in connection with the explanation of 'Requisite Conditions for Spawning,' given below, because some shallows contain more sand and fewer stones, and others contain many larger stones but no sand, while others contain pebbles lying over either rocks or sand. The lampreys remove some of the material, and if they do not find all the essentials for a spawning nest, the site is deserted and the creatures move on."
Requisite Conditions for Spawning with Lampreys.—"For a spawning site two conditions are immediately essential—proper conditions of water and suitable stream bed or bottom. Of course with these it is essential that no impassable barriers (dam or falls) exist between the lake and the spawning sites to prevent migration at the proper 'running' season. Lampreys will not spawn where there is no sand lying on the bottom between the rocks, as sand is essential in covering the eggs (see remarks on the 'Spawning Process'); neither will they spawn where the bottom is all sand and small gravel, as they cannot take hold of this material with their mouths to construct nests or to hold themselves in the current, and they would not find here pebbles and stones to carry over the nest while spawning, as described elsewhere. It can thus be seen that, as suggested above, the reason they do not spawn in Fall Creek and Cascadilla Creek, between the lake and the falls, is that the beds of these streams are very rocky, being covered only with large stones and no sand. There is no doubt that the lampreys find here suitable conditions of water, but they do not remain to spawn on account of the absence of the proper conditions of stream bed. Again, they do not spawn in the lower course of the inlet for a distance of nearly two miles from the lake, because near the lake the bed of the stream is composed of silt, while for some distance above this (up-stream) there is nothing but sand. Farther up-stream are found pebbles and stones commingled with sand, which combination satisfies the demands of the lampreys for material in constructing nests and covering eggs. The accessibility of these sites, together with their suitable conditions, render the inlet the great and perhaps the only spawning stream of the lake; and, doubtless, all the mature lampreys come here to spawn, excepting a few which spawn in the lower part of Six-mile Creek, a tributary of the inlet.
"As the course of the stream where the beds abound is divided into pools, separated by stony ripples or shallows, the nests must be made at the ends of the pools. Of the spawning beds personally observed during several seasons, nine-tenths of the entire number were formed just above the shallows at the lower ends of the pools, while only a few were placed below them. An advantage in forming the nest above the shoals rather than below it is that in the former place the water runs more swiftly over the lower and middle parts of such a bed than at its upper margin, since the velocity decreases in either direction from the steeper part of the shallows; and any organic material or sediment that would wash over the upper edge of the nest is thus carried on rather than left as a deposit. When formed below the shallows, owing to the decreased velocity at the lower part of the nest compared with that at the upper, the sediment is likely to settle in the hollow of the nest, and, through the process of decay of the organic material, prove disastrous or unfavorable for the developing embryos.
"The necessity of sand in the spawning bed indicates the explanation of why we see so many shallows which have no spawning lampreys upon them, while there are others in the same vicinity that are crowded. There will be no nests formed if there is too little or too much sand, not enough or too many stones, or stones that are all too small or all too large. The stones must vary from the size of an egg to the size of a man's hand, and must be intermingled with sand without mud or rubbish.
"The lampreys choose to make their spawning nests just where the water flows so swiftly that it will carry the sand a short distance, but will not sweep it out of the nest. This condition furnishes not only force to wash the sand over the eggs when laid, but also keeps the adult lampreys supplied with an abundance of fresh water containing the dissolved air needed for their very rapid respiration. Of course in such rapid water the eggs are likely to be carried away down-stream, but Nature provides against this by the fact that they are adhesive, and the mating lampreys stir up the sand with their tails, thus weighing down the freshly laid eggs and holding them in the nest. Hence the necessity of an abundance of sand at the spawning site."
The Spawning Process with Lampreys.—"There is much interest in the study of the spawning process, as it is for the maintenance of the race that the lampreys risk and end their lives; and as they are by far the lowest form of vertebrates found within the United States, a consideration of their actions and apparent evidences of instinct becomes of unusual attraction. Let us consider one of those numerous examples in which the male migrates before the female. When he comes to that portion of the stream where the conditions named above are favorable, he commences to form a nest by moving and clearing stones and making a basin with a sandy bottom about the size of a common wash-bowl. Several nests may be started and deserted before perfect conditions are found for the completion of one. The male may be joined by a female either before or after the nest is completed. There is at once harmony in the family; but if another male should attempt to intrude, either before or after the coming of the female, he is likely to be summarily dealt with and dismissed at once by the first tenant. As soon as the female arrives she too commences to move pebbles and stones with her mouth.
"Sometimes the nest is made large enough to contain several pairs, or often unequal numbers of males and females; or they may be constructed so closely together as to form one continuous ditch across the stream, just above the shallows. Many stones are left at the sides and especially at the upper margin of the nest, and to these both lampreys often cling for a few minutes as though to rest. While the female is thus quiet, the male seizes her with his mouth at the back of her head, clinging as to a fish. He presses his body as tightly as possible against her side, and loops his tail over her near the vent and down against the opposite side of her body so tightly that the sand, accidentally coming between them, often wears the skin entirely off of either or both at the place of closest contact. In most observed instances the male pressed against the right side of the female, although there is no unvarying rule as to position. The pressure of the male thus aids to force the eggs from the body of the female, which flow very easily when ripe. The vents of the two lampreys are thus brought into close proximity, and the conspicuous genital papilla of the male serves to guide the milt directly to the issuing spawn. There appears to be no true intromission, although definite observation of this feature is quite difficult, and, in fact, impossible. During the time of actual pairing, which lasts but a few seconds, both members of the pair exhibit tremendous excitement, shaking their bodies in rapid vibrations and stirring up such a cloud of sand with their tails that their eggs are at once concealed and covered. As the eggs are adhesive and non-buoyant, the sand that is stirred up adheres to them immediately and covers most of them before the school of minnows in waiting just below the nest can dart through the water and regale themselves upon the eggs of these enemies of their race; but woe to the eggs that are not at once concealed. We would suggest that the function of the characteristic anal fin, which is possessed only by the female, and only at this time of year, may be to aid in this vastly important process of stirring up the sand as the eggs are expelled; and the explanation of the absence of such a fin from the ventral side of the tail of the male may be found in the fact that it could not be used for the same purpose at the instant when most needed, since the male is just then using his tail as a clasping organ to give him an essential position in pairing. As soon as they shake together they commence to move stones from one part of the nest to another, to bring more loose sand down over their eggs. They work at this from one to five minutes, then shake again, thus making the intervals between mating from one to five minutes, with a general average of about three and a half minutes.
"Although their work of moving stones does not appear to be systematic in reference to the placing of the pebbles, or as viewed from the standpoint of man, it does not need to be so in order to perfectly fulfill all the purposes of the lampreys. As shown above in the remarks on the spawning habits of the brook lampreys, the important end which they thus accomplish is the loosening and shifting of the sand to cover their eggs; and the more the stones are moved, even in the apparently indiscriminate manner shown, the better is this purpose achieved. Yet, in general, they ultimately accomplish the feat of moving to the lower side of the nest all the stones they have placed or left at the upper margin. At the close of the spawning season when the nest is seen with no large pebbles at its upper margin, but quite a pile of stones below, it can be known that the former occupants completed their spawning process there; but if many small stones are left at the upper edge and at the sides, and a large pile is not formed at the lower edge, it can be known that the nest was forsaken or the lampreys removed before the spawning process was completed. The stones they move are often twice as heavy as themselves, and are sometimes even three or four times as heavy. Since they are not attempting to build a stone wall of heavy material, there is no occasion for their joining forces to remove stones of extraordinary size, and they rarely do so, although once during the past spring (1900) we saw two lake lampreys carrying the same large stone down-stream across their nest. Although this place was occupied by scores of brook lampreys, there were but three pairs of lake lampreys seen here. It is true that one of these creatures often moves the same stone several times, and many even attempt many times to move a stone that has already been found too heavy for it; but sooner or later the rock may become undermined so that the water will aid them, and they have no way of knowing what they can do under such circumstances until they try. Also, the repeated moving of one stone may subserve the same purpose for the lamprey in covering its eggs with sand as would the less frequent removal of many.
"When disturbed on the spawning nest, either of the pair will return to the same nest if its mate is to be found there; but if its mate is in another place, it will go to it, and if its mate is removed or killed, it is likely to go to any part of the stream to another nest. When disturbed, they often start up-stream for a short distance, but soon dart down-stream with a velocity that is almost incredible. They can swim faster than the true fishes, and after they get a start are generally pretty sure to make good their escape, although we have seen them dart so wildly and frantically down-stream that they would shoot clear out on the bank and become an easy victim of the collector. This peculiar kind of circumstance is most likely to happen with those lampreys that are becoming blinded from long exposure to the bright light over the clear running water. If there is a solitary individual on a nest when disturbed, it may not return to that nest, but to any that has been started, or it may stay in the deep pool below the shallows until evening and then move some distance up-stream. When the nest is large and occupied by several individuals, those that are disturbed may return to any other such nest. We have never seen evidence of one female driving another female out of a spawning-nest; and from the great number of nests in which we have found the numbers of the females exceeding those of the males, we would be led to infer that the former live together in greater harmony than do the males.
"Under the subject of the number of eggs laid, we should have said that at one shake the female spawns from twenty to forty. We once caught in fine gauze twenty-eight eggs from a female at one spawning instant. In accordance with the frequency of spawning stated, and the number of eggs contained in the body of one female, the entire length of time given to the spawning process would be from two to four days. This agrees with the observed facts, although the lampreys spend much time in moving stones and thoroughly covering the nests with sand. Even after the work of spawning and moving stones is entirely completed, they remain clinging to rocks in various parts of the stream, until they are weakened by fungus and general debility, when they gradually drift down-stream.
"In forming nests there is a distinct tendency to utilize those sites that are concealed by overhanging bushes, branches, fallen tree-tops, or grass or weeds, probably not only for concealment, but also to avoid the bright sunlight, which sooner or later causes them to go blind, as it does many fishes when they have to live in water without shade. Toward the end of the spawning season, it is very common to see blind lampreys clinging helplessly to any rocks on the bottom, quite unable to again find spawning-beds. However, at such times they are generally spent and merely awaiting the inevitable end.
"As with the brook lamprey, the time of spawning and duration of the nesting period depend upon the temperature of the water, as does also the duration of the period of hatching or development of the embryo. They first run up-stream when the water reaches a temperature of 45° or 48° Fahr., and commence spawning at about 50°. A temperature of 60° finds the spawning process in its height, and at 70° it is fairly completed. It is thus that the rapidity with which the water becomes heated generally determines the length of time the lampreys remain in the stream. This may continue later in the season for those that run later, but usually it is about a month or six weeks from the time the first of this species is seen on a spawning-nest until the last is gone."
What becomes of Lampreys after Spawning?—"There has been much conjecture as to the final end of the lampreys, some writers contending that they die after spawning, others that they return to deep water and recuperate, and yet others compromise these two widely divergent views by saying that some die and others do not. The fact is that the spawning process completely wears out the lampreys, and leaves them in a physical condition from which they could never recover. They become stone-blind; the alimentary canal suffers complete atrophy; their flesh becomes very green from the katabolic products, which find the natural outlet occluded; they lose their rich yellow color and plump, symmetrical appearance; their skin becomes torn, scratched, and worn off in many places, so that they are covered with sores, and they become covered with a parasitic or sarcophytic fungus, which forms a dense mat over almost their entire bodies, and they are so completely debilitated and worn out that recovery is entirely out of the question. What is more, the most careful microscopical examination of ovaries and testes has failed to reveal any evidence of new gonads or reproductive bodies. This is proof that reproduction could not again ensue without a practical rebuilding of the animals, even though they should regain their vitality. A. Mueller, in 1865, showed that all the ova in the lamprey were of the same size, and that after spawning no small reproductive bodies remained to be developed later. This is strong evidence of death after once spawning.
"One author writes that an argument against the theory of their dying after spawning can be found in the fact that so few dead ones have been found by him. However, many can be found dead if the investigator only knows how and where to look for them. We should not anticipate finding them in water that is shallow enough for the bottom to be plainly seen, as there the current is strong enough to move them. It is in the deep, quiet, pools where sediment is depositing that the dead lampreys are dropped by the running water, and there they sink into the soft ooze.
"The absence of great numbers of dead lampreys from visible portions of the stream cannot be regarded as important evidence against the argument that they die soon after spawning once, as the bodies are very soon disintegrated in the water. In the weir that we maintained in 1898, a number of old, worn-out, and fungus-covered lampreys were caught drifting down-stream; some were dead, some alive, and others dying and already insensible, but none were seen going down that appeared to be in condition to possibly regain their strength."
Fig. 297a.—Brook Lamprey, Lampetra Wilderi. (After Gage.)
[CHAPTER XXIX]
THE CLASS ELASMOBRANCHII OR SHARK-LIKE FISHES
The Sharks.—The gap between the lancelets and the lampreys is a very wide one. Assuming the primitive nature of both groups, this gap must represent the period necessary for the evolution of brain, skull, and elaborate sense organs. The interspace between the lampreys and the nearest fish-like forms which follow them in an ascending scale is not less remarkable. Between the lamprey and the shark we have the development of paired fins with their basal attachments of shoulder-girdle and pelvis, the formation of a lower jaw, the relegation of the teeth to the borders of the mouth, the development of separate vertebræ along the line of the notochord, the development of the gill-arches, and of an external covering of enameled points or placoid scales.
These traits of progress separate the Elasmobranchs from all lower vertebrates. For those animals which possess them, the class name of Pisces or fishes has been adopted by numerous authors. If this term is to be retained for technical purposes, it should be applied to the aquatic vertebrates above the lampreys and lancelets. We may, however, regard fish as a popular term only, rather than to restrict the name to members of a class called Pisces. From the bony fishes, on the other hand, the sharks are distinguished by the much less specialization of the skeleton, both as regards form and substance, by the lack of membrane bones, of air-bladder, and of true scales, and by various peculiarities of the skeleton itself. The upper jaw, for example, is formed not of maxillary and premaxillary, but of elements which in the lower fishes would be regarded as belonging to the palatine and pterygoid series. The lower jaw is formed not of several pieces, but of a cartilage called Meckel's cartilage, which in higher fishes precedes the development of a separate dentary bone. These structures are sometimes called primary jaws, as distinguished from secondary jaws or true jaws developed in addition to those bones in the Actinopteri or typical fishes. In the sharks the shoulder-girdle is attached, not to the skull, but to a vertebra at some distance behind it, leaving a distinct neck, such as is possessed or retained by the vertebrate higher than fishes. The shoulder-girdle itself is a continuous arch of cartilage, joining its fellow at the breast of the fish. Other peculiar traits will be mentioned later.
Characters of Elasmobranchs.—The essential character of the Elasmobranchs as a whole are these: The skeleton is cartilaginous, the skull without sutures, and the notochord more or less fully replaced or inclosed by vertebral segments. The jaws are peculiar in structure, as are also the teeth, which are usually highly specialized and found on the jaws only. There are no membrane bones; the shoulder-girdle is well developed, each half of one piece of cartilage, and the ventral fins, with the pelvic-girdle, are always present, always many-rayed, and abdominal in position. The skin is covered with placoid scales, or shagreen, or with bony bucklers, or else it is naked. It is never provided with imbricated scales. The tail is diphycercal, heterocercal, or else it degenerates into a whip-like organ, a form which has been called leptocercal. The gill-arches are 5, 6, or 7 in number, with often an accessory gill-slit or spiracle. The ventral fins in the males (except perhaps in certain primitive forms) are provided with elaborate cartilaginous appendages or claspers. The brain is elongate, its parts well separated, the optic nerves interlacing. The heart has a contractile arterial cone containing several rows of valves; the intestine has a spiral valve; the eggs are large, hatched within the body, or else deposited in a leathery case.
Classification of Elasmobranchs.—The group of sharks and their allies, rays, and Chimæras, is usually known collectively as Elasmobranchii (ἐλάσμος, blade or plate; βράγχος, gill). Other names applied to all or a part of this group are these: Selachii (σελαχός, a cartilage, the name also used by the Greeks for the gristle-fishes or sharks); Plagiostomi (πλαγιός, oblique; στόμα, mouth); Chondropterygii (χόνδρος, cartilage; πτερύξ, fin); and Antacea (ἀντακαῖος, sturgeon). They represent the most primitive known type of jaw-bearing vertebrates, or Gnathostomi (γνάθος, jaw; στόμα, mouth), the Chordates without jaws being sometimes called collectively Agnatha (ἀ-γνάθος, without jaws). These higher types of fishes have been also called collectively Lyrifera, the form of the two shoulder-girdles taken together being compared to that of a lyre. Through shark-like forms all the higher vertebrates must probably trace their descent. Sharks' teeth and fin-spines are found in all rocks from the Upper Silurian deposits to the present time, and while the majority of the genera are now extinct, the class has had a vigorous representation in all the seas, later Palæozoic, Mesozoic, and Cenozoic, as well as in recent times.
Most of the Elasmobranchs are large, coarse-fleshed, active animals feeding on fishes, hunting down their prey through superior strength and activity. But to this there are many exceptions, and the highly specialized modern shark of the type of the mackerel-shark or man-eater is by no means a fair type of the whole great class, some of the earliest types being diminutive, feeble, and toothless.
Subclasses of Elasmobranchs.—With the very earliest recognizable remains it is clear that the Elasmobranchs are already divided into two great divisions, the sharks and the Chimæras. These groups we may call subclasses, the Selachii and the Holocephali, or Chismopnea.
The Selachii, or sharks and rays, have the skull hyostylic, that is, with the quadrate bone grown fast to the palate which forms the upper jaw, the hyomandibular, acting as suspensorium to the lower jaw, being articulated directly to it.
The palato-quadrate apparatus, the front of which forms the upper jaw in the shark, is not fused to the cranium, although it is sometimes articulated with it. There are as many external gill-slits as there are gill-arches (5, 6, or 7), and the gills are adnate to the flesh of their own arches, without free tips. The cerebral hemispheres are grown together. The teeth are separated and usually strongly specialized, being primitively modified from the prickles or other defences of the skin. There is no frontal holder or bony hook on the forehead of the male.
The subclass Holocephali, or Chimæras, differ from the sharks in all this series of characters, and its separation as a distinct group goes back to the Devonian or even farther, the earliest known sharks having little more in common with Chimæras than the modern forms have.
The Selachii.—There have been many efforts to divide the sharks and rays into natural orders. Most writers have contented themselves with placing the sharks in one order (Squali or Galei or Pleurotremi) having the gill-openings on the side, and the rays in another (Rajæ, Batoidei, Hypotrema) having the gill-openings underneath. Of far more importance than this superficial character of adaptation are the distinctions drawn from the skeleton. Dr. Gill has used the attachment of the palato-quadrate apparatus as the basis of a classification. The Opistharthri (Hexanchidæ) have this structure articulated with the postorbital part of the skull. In the Prosarthri (Heterodontidæ) it is articulated with the preorbital part of the skull, while in the other sharks (Anarthri) it is not articulated at all. But these characters do not appear to be always important. Chlamydoselachus, for example, differs in this regard from Heptranchias, which in other respects it closely resembles. Yet, in general, the groups thus characterized are undoubtedly natural ones.
Fig. 298.—Fin-spine of Onchus tenuistriatus Agassiz. (After Zittel.)
Hasse's Classification of Elasmobranchs.—In 1882, Professor Carl Hasse proposed to subdivide the sharks on the basis of the structure of the individual vertebræ. In the lowest group, a hypothetical order of Polyospondyli, possibly represented by the fossil spines called Onchus, an undivided notochord, perhaps swollen at regular intervals, is assumed to have represented the vertebral column. In the Diplospondyli (Hexanchidæ) the imperfectly segmented vertebræ are joined in pairs, each pair having two neural arches. In the Asterospondyli or ordinary sharks each vertebra has its calcareous lamella radiating star-like from the central axis. In the Cyclospondyli (Squalidæ, etc.) the calcareous part forms a single ring about the axis, and in the Tectospondyli (Squatina, rays, etc.) it forms several rings. These groups again are natural and correspond fairly with those based on other characters. At the same time there is no far-reaching difference between Cyclospondyli and Tectospondyli, and the last-named section includes both sharks and rays.
Fig. 299.—Section of vertebræ of sharks, showing calcification. (After Hasse.) 1. Cyclospondyli (Squalus); 2. Tectospondyli (Squatina); 3. Asterospondyli (Carcharias).
Nothing is known of the Polyospondyli, and they may never have existed at all. The Diplospondyli do not differ very widely from the earlier Asterospondyli (Cestraciontes) which, as a matter of fact, have preceded the Diplospondyli in point of time, if we can trust our present knowledge of the geological record.
Other Classifications of Elasmobranchs.—Characters more fundamental may be drawn from the structure of the pectoral fin. In this regard four distinct types appear. In Acanthoessus this fin consists of a stout, stiff spine, with a rayless membrane attached behind it. In Cladoselache the fin is low, with a very long base, like a fold of skin (ptychopterygium), and composed of feeble rays. In Pleuracanthus it is a jointed axis of many segments, with a fringe of slender fin-rays, corresponding in structure to all appearance to the pectoral fin of Dipnoans and Crossopterygians, the type called by Gegenbaur archipterygium on the hypothesis that it represents the primitive vertebrate limb.
In most sharks the fin has a fan-shape, with three of the basal segments larger than the others. Of these the mesopterygium is the central one, with the propterygium before it and the metapterygium behind. In the living sharks of the family of Heterodontidæ, this form of fin occurs and the teeth of the same general type constitute the earliest remains distinctly referable to sharks in the Devonian rocks.
Primitive Sharks.—Admitting that these four types of pectoral fin should constitute separate orders, we have next to consider which form is the most primitive and what is the line of descent. In this matter we have, in the phrase of Hæckel, only the "three ancestral documents, Palæontology, Morphology, and Ontogeny."
Unfortunately the evidence of these documents is incomplete and conflicting. So far as Palæontology is concerned, the fin of Cladoselache, with that of Acanthoessus, which may be derived from it, appears earliest, but the modern type of pectoral fin with the three basal segments is assumed to have accompanied the teeth of Psammodonts and Cochliodonts, while the fin of the Chimæra must have been developed in the Devonian. The jointed fin of Cladodus and Pleuracanthus may be a modification or degradation of the ordinary type of shark-fin.
Assuming, however, that the geological record is not perfect and that the fin of Cladoselache is not clearly shown to be primitive, we have next to consider the evidence drawn from morphology.
Those who with Balfour and others (see page [69]) accept the theory that the paired fins are derived from a vertebral fold, will regard with Dean the fin of Cladoselache as coming nearest the theoretical primitive condition.
The pectoral fin in Acanthoessus Dean regards as a specialized derivative from a fin like that of Cladoselache, the fin-rays being gathered together at the front and joined together to form the thick spine characteristic of Acanthoessus. This view of the morphology of the fin of Acanthoessus is not accepted by Woodward, and several different suggestions have been recorded.
If with Gegenbaur we regard the paired fins as derived from the septa between the gill-slits, or with Kerr regard them as modified external gills, the whole theoretical relation of the parts is changed. The archipterygium of Pleuracanthus would be the nearest approach to the primitive pectoral limb, and from this group and its allies all the other sharks are descended. This central jointed axis of Pleuracanthus is regarded by Traquair as the equivalent of the metapterygium in ordinary sharks. (See Figs. 44, 45, 46.)
According to Traquair: "The median stern [of the archipterygium], simplified, shortened up and losing all its radials on the postaxial side, except in sometimes a few near the tip, becomes the metapterygium, while the mesopterygium and propterygium are formed by the fusion into two pieces of the basal joints of a number of preaxial radials, which have reached and become attached to the shoulder-girdle in front of the metapterygium."
According to Dr. Traquair, the pectoral fin in Cladodus neilsoni, a shark from the Coal Measures of Scotland, is "apparently a veritable uniserial archipterygium midway between the truly biserial one of Pleuracanthus and the pectoral fin of ordinary sharks." Other authors look on these matters differently, and Dr. Traquair admits that an opposite view is almost equally probable. Cope and Dean would derive the tribasal pectoral of ordinary sharks directly from the ptychopterygium or fan-like fold of Cladoselache, while Fritsch and Woodward would look upon it as derived in turn from the Ceratodus-like fin of Pleuracanthus, itself derived from the ptychopterygium or remains of a lateral fin-fold.
If the Dipnoans are descended from the Crossopterygians, as Dollo has tried to show, the archipterygium of Pleuracanthus has had a different origin from the similar-appearing limb of the Dipnoans, Dipterus and Ceratodus.
In such case the archipterygium would not be the primitive pectoral limb, but a structure which may have been independently evolved in two different groups.
In the view of Gegenbaur, the Crossopterygians and Dipnoans with all the higher vertebrates and the bony fishes would arise from the same primitive stock, ancestors, or allies of the Ichthyotomi, which group would also furnish the ancestors of the Chimæras. In support of this view, the primitive protocercal or diphycercal tail of Pleuracanthus may be brought in evidence as against the apparently more specialized heterocercal tail of Cladoselache. But this is not conclusive, as the diphycercal tail may arise separately in different groups through degeneration, as Dollo and Boulenger have shown.
The matter is one mainly of morphological interpretation, and no final answer can be given. On page [68] a summary of the various arguments may be found. Little light is given by embryology. The evidence of Palæontology, so far as it goes, certainly favors the view of Balfour. Omitting detached fin-spines and fragments of uncertain character, the earliest identifiable remains of sharks belong to the lower Devonian. These are allies of Acanthoessus. Cladoselache comes next in the Upper Devonian. Pleuracanthus appears with the teeth and spines supposed to belong to Cestraciont sharks, in the Carboniferous Age. The primitive-looking Notidani do not appear before the Triassic. For this reason the decision as to which is the most primitive type of shark must therefore rest unsettled for the present and perhaps for a long time to come.
The weight of authority at present seems to favor the view of Balfour, Wiedersheim, Boulenger, and Dean, that the pectoral limb has arisen from a lateral fold of skin. But weight of authority is not sufficient when evidence is confessedly lacking.
For our purpose, without taking sides in this controversy, we may follow Dean in allowing Cladoselache to stand as the most primitive of known sharks, thus arranging the Elasmobranchs and rays, recent and fossil, in six orders of unequal value—Pleuropterygii, Acanthodei, Ichthyotomi, Notidani, Asterospondyli, and Tectospondyli. Of these orders the first and second are closely related, as are also the fourth and fifth, the sixth being not far remote. The true sharks form the culmination of one series, the rays of another, while from the Ichthyotomi the Crossopterygians and their descendants may be descended. But this again is very hypothetical, or perhaps impossible; while, on the other hand, the relation of the Chimæras to the sharks is still far from clearly understood.
Order Pleuropterygii.—The order of Pleuropterygii of Dean (πλεύρον, side; πτερύξ, fin), called by Parker and Haswell Cladoselachea, consists of sharks in which the pectoral and ventral fins have each a very wide horizontal base (ptychopterygium), without jointed axis and without spine. There are no spines in any of the fins. The dorsal fin is low, and there were probably two of them. The notochord is persistent, without intercalary cartilage, such as appear in the higher sharks. The caudal fin is short, broad, and strongly heterocercal. Apparently the ventral fin is without claspers. The gill-openings were probably covered by a dermal fold. The teeth are weak, being modified denticles from the asperities of the skin. The lateral line is represented by an open groove. The family of Cladoselachidæ consists of a single genus Cladoselache from the Cleveland shale or Middle Devonian of Ohio. Cladoselache fyleri is the best-known species, reaching a length of about two feet. Dean regards this as the most primitive of the sharks, and the position of the pectorals and ventrals certainly lend weight to Balfour's theory that they were originally derived from a lateral fold of skin. I am recently informed by Dr. Dean that he has considerable evidence that in Cladoselache the anus was subterminal. If this statement is verified, it would go far to establish the primitive character of Cladoselache.
Fig. 300.—Cladoselache fyleri (Newberry), restored. Upper Devonian of Ohio. (After Dean.)
Order Acanthodei.—Near the Pleuropterygii, although much more highly developed, we may note the strange group of Acanthodei (ἀκανθώδης, spinous). These armed fishes were once placed among the Crossopterygians, but there seems no doubt that Woodward is right in regarding them as a highly specialized aberrant offshoot of the primitive sharks. In this group the paired fins consist each of a single stout spine, nearly or quite destitute of other rays. A similar spine is placed in front of the dorsal fin and one in front of the anal. According to Dean these spines are each produced by the growing together of all the fin-rays normally belonging to the fin, a view of their morphology not universally accepted.
Fig. 301.—Cladoselache fyleri (Newberry), restored. Ventral view. (After Dean.)
Fig. 302.—Teeth of Cladoselache fyleri (Newberry). (After Dean.)
Fig. 303.—Acanthoessus wardi (Egerton). Carboniferous. Family Acanthoessidæ. (After Woodward.)
The dermal covering is highly specialized, the shagreen denticles being much enlarged and thickened, often set in little squares suggesting a checker-board. The skull is covered with small bony plates and membrane bones form a sort of ring about the eye. The teeth are few, large, and "degenerate in their fibrous structure." Some of the species have certainly no teeth at all. The tail is always heterocercal, or bent upward at tip as in the Cladoselache, not diphycercal, tapering and horizontal as in the Ichthyotomi.
The lower Acanthodeans, according to Woodward, "are the only vertebrates in which there are any structures in the adult apart from the two pairs of fins which may be plausibly interpreted as remnants of once continuous lateral folds. In Climatius, one of the most primitive genera (see Fig. 305), there exists, according to Woodward, and as first noticed by Cope, between the pectoral and pelvic (or ventral) fins a close and regular series of paired spines, in every respect identical with those supporting the appendages that presumably correspond to the two pairs of fins in the higher genera. They may even have supported fin membranes, though specimens sufficiently well preserved to determine this point have not yet been discovered. However, it is evident that dermal calcifications attained a greater development in the Acanthodei than in any of the more typical Elasmobranchs, and we may look for much additional information on the subject when the great fishes to which the undetermined Ichthyodorulites pertained became known." (See Fig. 305.)
The Acanthodei constitute three families. In the Acanthoessidæ there is but one short dorsal fin opposite the anal, and clavicular bones are absent. The gill-openings being provided with "frills" or collar-like margins, perhaps resembled those of the living genus Chlamydoselachus, the frilled shark. The pectoral spine is very strong, and about the eye is a ring of four plates. The body is elongate, tapering, and compressed. Acanthoessus of Agassiz, the name later changed by its author to Acanthodes, is the principal genus, found in the Devonian and Carboniferous.
The species of Acanthoessus are all small fishes rarely more than a foot long, with very small teeth or none, and with the skin well armed with a coat-of-mail. Acanthoessus bronni is the one longest known. In the earliest species known, from the Devonian, the ventral fins are almost as large as the pectorals and nearly midway between pectorals and anal. In the later species the pectoral fins become gradually larger and the ventrals move forward. In the Permian species the pectorals are enormous.
Traquairia pygmæa, from the Permian of Bohemia, is a diminutive sharklet three or four inches long with large scales, slender spines, and apparently no ventral fins.
In the genus Cheiracanthus the dorsal fin is placed before the anal. In Acanthodopsis the teeth are few, large, and triangular, and the fin-spines relatively large.
The Ischnacanthidæ have no clavicles, and two dorsal fins. Ischnacanthus gracilis of the Devonian has a few large conical teeth with small cusps between them.
The Diplacanthidæ, with two dorsal fins, possess bones interpreted as clavicles. The teeth are minute or absent. In Diplacanthus striatus and Diplacanthus longispinus of the Lower Devonian stout spines are attached to the shoulder-girdle between the pectoral spines below.
Fig. 304.—Diplacanthus crassissimus Duff. Devonian. Family Diplacanthidæ. (After Nicholson). (Restoration of jaws and gill-openings; after Traquair.)
In the very small sharks called Climatius the fin-spines are very strong, and a series of several free spines occurs, as above stated, on each side between the pectoral and ventral fins, a supposed trace of a former lateral fold. In Paraxus the first dorsal spine is enormously enlarged in size, the other spines remaining much as in Climatius.
Dean on Acanthodei.—In his latest treatise on these fishes, "The Devonian Lamprey," Dr. Dean unites the Pleuropterygii and Acanthodei in a single order under the former name, regarding Acanthoessus as an ally and perhaps descendant of the primitive Cladoselache. Dr. Dean observes:
"In the foregoing classification it will be noted that the Acanthodia are regarded as included under the first order of sharks, Pleuropterygii. To this arrangement Smith Woodward has already objected that the spines of Acanthodians cannot be regarded as the homologues of the radial elements of the Cladoselachian fin (which by a process of concrescence have become fused in its interior margin), since he believes the structure to be entirely dermal in origin. His criticism, however, does not seem to me to be well grounded, for, although all will admit that Acanthodian spines have become incrusted, and deeply incrusted, with a purely dermal calcification, it does not follow that the interior of the spine has not had primitively a non-dermal core. That the concrescence of the radial supporting elements of the fin took place pari passu with the development of a strengthening dermal support of the fin margin was the view expressly formulated in my previous paper on this subject. It is of interest in this connection to recall that the earliest types of Acanthodian spines were the widest, and those which, in spite of their incasing dermal calcification, suggest most clearly the parallel elements representing the component radial supports. There should also be recalled the many features in which the Acanthodians have been shown to resemble Cladoselache."
Fig. 305.—Climatius scutiger Egerton, restored. Family Diplacanthidæ. (After Powrie, per Zittel.)
From these primitive extinct types of shark we may proceed to those forms which have representatives among living fishes. From Cladoselache a fairly direct series extends through the Notidani and Cestraciontes, culminating in the Lamnoid and Galeoid sharks.
Still another series, destitute of anal fin, probably arising near the Acanthodei, reaches its highest development in the side branch of the Batoidei or rays. The Holocephali and Dipneusti must also find their origin in some of these primitive types, certainly not in any form of more highly specialized sharks.
Fig. 306.—Pleuracanthus decheni Goldfuss. Family Pleuracanthidæ. (After Roemer, per Zittel.)
Woodward prefers to place the Tectospondyli next to the Ichthyotomi, leaving the specialized sharks to be treated later. There is, however, no linear system which can interpret natural affinities, and we follow custom in placing the dogfishes and rays at the end of the shark series.
Fig. 307.—Pleuracanthus decheni, restored. (After Brongniart.) The anterior anal very hypothetical.
Fig. 308.—Head-bones and teeth of Pleuracanthus decheni Goldfuss. (After Davis, per Dean.)
Fig. 309.—Teeth of Didymodus bohemicus Quenstadt. Carboniferous. Family Pleuracanthidæ. (After Zittel)
Order Ichthyotomi.—In the order Ichthyotomi (ἰχθύς, fish; τομός, cutting; named by Cope from the supposed segmentation of the cranium; called by Parker and Haswell Pleuracanthea) the very large pectoral fins are developed each as an archipterygium. Each fin consists of a long segmented axis fringed on one or both sides with fin-rays. The notochord is very simple, scarcely or never constricted, the calcifications of its sheath "arrested at the most primitive or rhachitomous stage, except in the tail." This is the best defined of the orders of sharks, and should perhaps rank rather as a subclass, as the Holocephali. Two families of Ichthyotomi are recognized by Woodward, the Pleuracanthidæ and the Cladodontidæ. In the Pleuracanthidæ the dorsal fin is long and low, continuous from head to tail, and the pectoral rays are in two rows. There is a long barbed spine with two rows of serrations at the nape. The body is slender, not depressed, and probably covered with smooth skin. The teeth have two or more blunt cusps, sometimes with a smaller one between and a blunt button behind. The interneural cartilages are more numerous than the neural spines. The genera are imperfectly known, the skeleton of Pleuracanthus decheni only being well preserved. This is the type of the genus called Xenacanthus which, according to Woodward, is identical with Pleuracanthus, a genus otherwise known from spines only. The denticles on the spine are straight or hooked backward, in Pleuracanthus (lævissimus), the spine being flattened. In Orthacanthus (cylindricus), the spine is cylindrical in section. The species called Dittodus and Didymodus are known from the teeth only. These resemble the teeth of Chlamydoselachus. It is not known that Dittodus possesses the nuchal spine, although detached spines like those of Pleuracanthus lie about in remains called Didymodus in the Permian rocks of Texas. In Dicranodus texensis the palato-quadrate articulates with the postorbital process of the cranium, as in the Hexanchidæ, and the hyomandibular is slender.
Fig. 310.—Shoulder-girdle and pectoral fins of Cladodus neilsoni Traquair.
A genus, Chondrenchelys, from the sub-Carboniferous of Scotland, is supposed to belong to the Pleuracanthidæ, from the resemblance of the skeleton. It has no nuchal spine, and no trace of paired fins is preserved.
The Cladodontidæ differ in having the "pectoral fin developed in the form of a uniserial archipterygium intermediate between the truly biserial one of Pleuracanthus and the pectoral fin of modern sharks." The numerous species are known mainly from detached teeth, especially abundant in America, the earliest being in the Lower Carboniferous. One species, Cladodus nelsoni (Fig. 310), described by Traquair, from the sub-Carboniferous of Scotland shows fairly the structure of the pectoral fin.
Fig. 311.—Teeth of Cladodus striatus Agassiz. (After Davis.) Carboniferous.
In Cladodus mirabilis the teeth are very robust, the crown consisting of a median principal cone and two or three large lateral cones on each side. The cones are fairly striate. In Lambdodus from Illinois there are no lateral cones. Other genera are Dicentrodus, Phœbodus, Carcharopsis, and Hybocladodus.
[CHAPTER XXX]
THE TRUE SHARKS
Order Notidani.—We may recognize as a distinct order, a primitive group of recent sharks, a group of forms finding its natural place somewhere between the Cladoselachidæ and Heterodontidæ, both of which groups long preceded it in geological time.
The name Notidani (Notidanus, νωτιδάνος, dry back, an old name of one of the genera) may be retained for this group, which corresponds to the Diplospondyli of Hasse, the Opistharthri of Gill, and the Protoselachii of Parker and Haswell. The Notidani are characterized by the primitive structure of the spinal column, which is without calcareous matter, the centra being imperfectly developed. There are six or seven branchial arches, and in the typical forms (not in Chlamydoselachus) the palato-quadrate or upper jaw articulates with the postorbital region of the skull. The teeth are of primitive character, of different forms in the same jaw, each with many cusps. The fins are without spines, the pectoral fin having the three basal cartilages (mesopterygium with propterygium and metapterygium) as usual among sharks.
Fig. 312.—Griset or Cow-shark, Hexanchus griseus (Gmelin). Currituck Inlet, N. C.
The few living forms are of high interest. The extinct species are numerous, but not very different from the living species.
Family Hexanchidæ.—The majority of the living Notidanoid sharks belong to the family of Hexanchidæ. These sharks have six or seven gill-openings, one dorsal fin, and a relatively simple organization. The bodies are moderately elongate, not eel-shaped, and the palato-quadrate articulates with the postorbital part of the skull. The six or eight species are found sparsely in the warm seas. The two genera, Hexanchus, with six, and Heptranchias, with seven vertebræ, are found in the Mediterranean. The European species are Hexanchus griseus, the cow-shark, and Heptranchias cinereus. The former crosses to the West Indies. In California, Heptranchias maculatus and Hexanchus corinus are occasionally taken, while Heptranchias deani is the well known Aburazame or oil shark of Japan. Heptranchias indicus, a similar species, is found in India.
Fig. 313.—Teeth of Heptranchias indicus Gmelin.
Fossil Hexanchidæ exist in large numbers, all of them referred by Woodward to the genus Notidanus (which is a later name than Hexanchus and Heptranchias and intended to include both these genera), differing chiefly in the number of gill-openings, a character not ascertainable in the fossils. None of these, however, appear before Cretaceous time, a fact which may indicate that the simplicity of structure in Hexanchus and Heptranchias is a result of degeneration and not altogether a mark of primitive simplicity. The group is apparently much younger than the Cestraciontes and little older than the Lamnoids, or the Squaloid groups. Heptranchias microdon is common in English Cretaceous rocks, and Heptranchias primigenius and other species are found in the Eocene.
Family Chlamydoselachidæ.—Very great interest is attached to the recent discovery by Samuel Garman of the frilled shark, Chlamydoselachus anguineus, the sole living representative of the Chlamydoselachidæ.
Fig. 314.—Frill-shark, Chlamydoselachus anguineus Garman. From Misaki, Japan. (After Günther.)
This shark was first found on the coast of Japan, where it is rather common in deep water. It has since been taken off Madeira and off the coast of Norway. It is a long, slender, eel-shaped shark with six gill-openings and the palato-quadrate not articulated to the cranium. The notochord is mainly persistent, in part replaced by feeble cyclospondylic vertebral centra. Each gill-opening is bordered by a broad frill of skin. There is but one dorsal fin. The teeth closely resemble those of Dittodus or Didymodus and other extinct Ichthyotomi. The teeth have broad, backwardly extended bases overlapping, the crown consisting of three slender curved cusps, separated by rudimentary denticles. Teeth of a fossil species, Chlamydoselachus lawleyi, are recorded by J. W. Davis from the Pliocene of Tuscany.
Order Asterospondyli.—The order of Asterospondyli comprises the typical sharks, those in which the individual vertebræ are well developed, the calcareous lamellæ arranged so as to radiate, star-fashion, from the central axis. All these sharks possess two dorsal fins and one anal fin, the pectoral fin is normally developed, with the three basal cartilages; there are five gill-openings, and the tail is heterocercal.
Fig. 315.—Bullhead-shark, Heterodontus francisci (Girard). San Pedro, Cal.
Suborder Cestraciontes.—The most ancient types may be set off as a distinct suborder under the name of Cestraciontes or Prosarthri.
Fig. 316.—Lower jaw of Heterodontus philippi. From Australia. Family Heterodontidæ. (After Zittel.)
These forms find their nearest allies in the Notidani, which they resemble to some extent in dentition and in having the palato-quadrate articulated to the skull although fastened farther forward than in the Notidani. Each of the two dorsal fins has a strong spine.
Fig. 317.—Teeth of Cestraciont Sharks. (After Woodward.) d, Synechodus dubrisianus Mackie; e, Heterodontus canaliculatus Egerton; f, Hybodus striatulus Agassiz. (After Woodward.)
Fig. 318.—Egg of Port Jackson Shark, Heterodontus philippi (Lacépède). (After Parker & Haswell.)
Family Heterodontidæ.—Among recent species this group contains only the family of Heterodontidæ, the bullhead sharks, or Port Jackson sharks. In this family the head is high, with usually projecting eyebrows, the lateral teeth are pad-like, ridged or rounded, arranged in many rows, different from the pointed anterior teeth, the fins are large, the coloration is strongly marked, and the large egg-cases are spirally twisted. All have five gill-openings. The living species of Heterodontidæ are found only in the Pacific, the Port Jackson shark of Australia, Heterodontus philippi, being longest known. Other species are Heterodontus francisci, common in California, Heterodontus japonicus, in Japan, and Heterodontus zebra, in China. These small and harmless sharks at once attract attention by their peculiar forms. In the American species the jaws are less contracted than in the Asiatic species, called Heterodontus. For this reason Dr. Gill has separated the former under the name of Gyropleurodus. The differences are, however, of slight value. The genus Heterodontus first appears in the Jurassic, where a number of species are known, one of the earliest being Heterodontus falcifer.
Three families of Cestraciontes are recognized by Hay. The most primitive of these is the group of Orodontidæ. Orodus, from the Lower Carboniferous, has the teeth with a central crown, its surface wrinkled. Of the Heterodontidæ, Hybodus, of the Carboniferous and Triassic, is one of the earliest and largest genera, characterized by elongate teeth of many cusps, different in different parts of the jaw, somewhat as in the Hexanchidæ, the median points being, however, always longest. The dorsal fins are provided with long spines serrated behind. The vertebræ with persistent notochord show qualities intermediate between those of Hexanchidæ and Heterodontidæ, and the same relation is shown by the teeth. In this genus two large hooked half-barbed dermal spines occur behind each orbit.
Fig. 319.—Tooth of Hybodus delabechei Charlesworth. (After Woodward.)
Fig. 320.—Fin-spine of Hybodus basanus Egerton. Cretaceous. Family Heterodontidæ. (After Nicholson.)
Fig. 321.—Fin-spine of Hybodus reticulatus Agassiz. (After Zittel.)
Palæospinax, with short stout spines and very large pectoral fins, formerly regarded as a dogfish, is placed near Heterodontus by Woodward. Acrodus, from the Triassic, shows considerable resemblance to Heterodontus. Its teeth are rounded and without cusps.
Most of these species belong to the Carboniferous, Triassic, and Jurassic, although some fragments ascribed to Cestraciont sharks occur in the Upper Silurian. Asteracanthus, known only from fin-spines in the Jura, probably belongs here.
It is a singular fact first noted by Dr. Hay, that with all the great variety of sharks, ten families in the Carboniferous age, representatives of but one family, Heterodontidæ, are found in the Triassic. This family may be the parent of all subsequent sharks and rays, six families of these appearing in the Jurassic and many more in the Cretaceous.
Edestus and its Allies.—Certain monstrous structures, hitherto thought to be fin-spines, are now shown by Dr. Eastman and others to be coalescent teeth of Cestraciont sharks.
Fig. 322.—Fin-spine of Hybodus canaliculatus Agassiz.
Fig. 323.—Teeth of Cestraciont Sharks. (After Woodward.) a, Hybodus lævis Woodward (after Woodward); b, Heterodontus rugosus Agassiz; c, Hybodus delabechei Charlesworth.
Fig. 324.—Edestus vorax Leidig, supposed to be a whorl of teeth. (After Newberry.)
These remarkable Ichthyodorulites are characteristic structures of sharks of unknown nature, but probably related to the Heterodontidæ. Of these the principal genera are Edestus, Helicoprion, and Campyloprion. Karpinsky regards these ornate serrated spiral structures as whorls of unshed teeth cemented together and extending outside the mouth, "sharp, piercing teeth which were never shed but became fused in whorls as the animals grew."
Dr. Eastman has, however, shown that these supposed teeth of Edestus are much like those of the Cochliodontidæ, and the animals which bore them should doubtless find their place among the Cestraciont sharks, perhaps within the family of Heterodontidæ.
Fig. 325.—Helicoprion bessonowi Karpinsky. Teeth from the Permian of Krasnoufimsk, Russia. (After Karpinsky.)
Onchus.—The name Onchus was applied by Agassiz to small laterally compressed spines, their sides ornamented with smooth or faintly crenulated longitudinal ridges, and with no denticles behind. Very likely these belonged to extinct Cestraciont sharks. Onchus murchisoni and Onchus tenuistriatus occur in the Upper Silurian rocks of England, in the lowest strata in which sharks have been found.
To a hypothetical group of primitive sharks Dr. Hasse has given the name of Polyospondyli. In these supposed ancestral sharks the vertebræ were without any ossification, a simple notochord, possibly swollen at intervals. The dorsal fin was single and long, a fold of skin with perhaps a single spine as an anterior support. The teeth must have been modified dermal papillæ, each probably with many cusps. Probably seven gill-openings were developed, and the tail was diphycercal, ending in a straight point. The finely striated fin-spines not curved upward at tip, called Onchus from the Upper Silurian of the Ludlow shales of England and elsewhere, are placed by Hasse near his Polyspondylous sharks. Such spines have been retained by the group of Chimæras, supposed to be derived from the ancestors of Onchus, as well as by the Heterodontidæ and Squalidæ.
Family Cochliodontidæ.—Another ancient family known from teeth alone is that of Cochliodontidæ. These teeth resemble those of the Heterodontidæ, but are more highly specialized. The form of the body is unknown, and the animals may have been rays rather than sharks. Eastman leaves them near the Petalodontidæ, which group of supposed rays shows a similar dentition. The teeth are convex in form, strongly arched, hollowed at base, and often marked by ridges or folds, being without sharp cusps. In each jaw is a strong posterior tooth with smaller teeth about. The elaborate specialization of these ancient teeth for crushing or grinding shells is very remarkable. The species are chiefly confined to rocks of the Carboniferous age. Among the principal genera are Helodus, Psephodus, Sandalodus, Venustodus, Xystrodus, Deltodus, Pœcilodus, and Cochliodus.
Fig. 326.—Lower jaw of Cochliodus contortus Agassiz. Carboniferous. (After Zittel.)
Concerning the teeth of various fossil sharks, Dr. Dean observes: "Their general character appears to have been primitive, but in structural details they were certainly specialized. Thus their dentition had become adapted to a shellfish diet, and they had evolved defensive spines at the fin margins, sometimes at the sides of the head. In some cases the teeth remain as primitive shagreen cusps on the rim of the mouth, but become heavy and bluntish behind; in other forms the fusion of tooth clusters may present the widest range in their adaptations for crushing; and the curves and twistings of the tritoral surfaces may have resulted in the most specialized forms of dentition which are known to occur, not merely in sharks but among all vertebrates."
In this neighborhood belongs, perhaps, the family of Tamiobatidæ, known from the skull of a single specimen, called Tamiobatis vetustus, from the Devonian in eastern Kentucky. The head has the depressed form of a ray, but it is probably a shark and one of the very earliest known.
Suborder Galei.—The great body of recent sharks belong to the suborder Galei, or Euselachii, characterized by the asterospondylous vertebræ, each having a star-shaped nucleus, and by the fact that the palato-quadrate apparatus or upper jaw is not articulated with the skull. The sharks of this suborder are the most highly specialized of the group, the strongest and largest and, in general, the most active and voracious. They are of three types and naturally group themselves about the three central families Scyliorhinidæ, Lamnidæ, and Carchariidæ (Galeorhinidæ).
The Asterospondyli are less ancient than the preceding groups, but the modern families were well differentiated in Mesozoic times.
Among the Galei the dentition is less complex than with the ancient forms, although the individual teeth are more highly specialized. The teeth are usually adapted for biting, often with knife-like or serrated edges; only the outer teeth are in function; as they are gradually lost, the inner teeth are moved outward, gradually taking the place of these.
We may place first, as most primitive, the forms without nictitating membrane.
Family Scyliorhinidæ.—The most primitive of the modern families is doubtless that of the Scyliorhinidæ, or cat-sharks. This group includes sharks with the dorsal fins both behind the ventrals, the tail not keeled and not bent upward, the spiracles present, and the teeth small and close-set. The species are small and mostly spotted, found in the warm seas. All of them lay their eggs in large cases, oblong, and with long filaments or strings at the corners. The cat-sharks, or roussettes, Scyliorhinus canicula and Catulus stellaris, abound in the Mediterranean. Their skin is used as shagreen or sandpaper in polishing furniture. The species of swell-sharks (Cephaloscylium) (C. uter, in California; C. ventriosus, in Chile; C. laticeps, in Australia; C. umbratile, in Japan) are short, wide-bodied sharks, which have the habit of filling the capacious stomach with air, then floating belly upward like a globefish. Other species are found in the depths of the sea. Scyliorhinus, Catulus, and numerous other genera are found fossil. The earliest is Palæoscyllium, in the Jurassic, not very different from Scyliorhinus, but the fins are described as more nearly like those of Ginglymostoma.
Close to the Scyliorhinidæ is the Asiatic family, Hemiscylliidæ, which differs in being ovoviviparous, the young, according to Mr. Edgar R. Waite, hatched within the body. The general appearance is that of the Scyliorhinidæ, the body being elongate. Chiloscyllium is a well-known genus with several species in the East Indies. Chiloscyllium modestum is the dogfish of the Australian fishermen. The Orectolobidæ are thick-set sharks, with large heads provided with fleshy fringes. Orectolobus barbatus (Crossorhinus of authors) abounds from Japan to Australia.
Another family, Ginglymostomidæ, differs mainly in the form of the tail, which is long and bent abruptly upward at its base. These large sharks, known as nurse-sharks, are found in the warm seas. Ginglymostoma cirrhatum is the common species with Orectolobus. Stegostoma tigrinum, of the Indian seas and north to Japan, one of several genera called tiger-sharks, is remarkable for its handsome spotted coloration. The extinct genus Pseudogaleus (voltai) is said to connect the Scyliorhinoid with the Carcharioid sharks.
The Lamnoid or Mackerel Sharks.—The most active and most ferocious of the sharks, as well as the largest and some of the most sluggish, belong to a group of families known collectively as Lamnoid, because of a general resemblance to the mackerel-shark, or Lamna, as distinguished from the blue sharks and white sharks allied to Carcharias (Carcharhinus).
The Lamnoid sharks agree with the cat-sharks in the absence of nictitating membrane or third eyelid, but differ in the anterior insertion of the first dorsal fin, which is before the ventrals. Some of these sharks have the most highly specialized teeth to be found among fishes, most effective as knives or as scissors. Still others have the most highly specialized tails, either long and flail-like, or short, broad, and muscular, fitting the animal for swifter progression than is possible for any other sharks. The Lamnoid families are especially numerous as fossils, their teeth abounding in all suitable rock deposits from Mesozoic times till now. Among the Lamnoid sharks numerous families must be recognized.
The most primitive is perhaps that of the Odontaspididæ (called Carchariidæ by some recent authors), now chiefly extinct, with the tail unequal and not keeled, and the teeth slender and sharp, often with smaller cusps at their base. Odontaspis and its relatives of the same genus are numerous, from the Cretaceous onward, and three species are still extant, small sharks of a voracious habit, living on sandy shores. Odontaspis littoralis (also known as Carcharias littoralis) is the common sand-shark of our Atlantic coast. Odontaspis taurus is a similar form in the Mediterranean.
Family Mitsukurinidæ, the Goblin-sharks.—Closely allied to Odontaspis is the small family of Mitsukurinidæ, of which a single living species is known. The teeth are like those of Odontaspis, but the appearance is very different.
The goblin-shark, or Tenguzame, Mitsukurina owstoni, is a very large shark rarely taken in the Kuro Shiwo, or warm "Black Current" of Japan. It is characterized by the development of the snout into a long flat blade, extending far beyond the mouth, much as in Polyodon and in certain Chimæras. Several specimens are now known, all taken by Capt. Alan Owston of Yokohama in Sagami Bay, Japan. The original specimen, a young shark just born, was presented by him to Professor Kakichi Mitsukuri of the University of Tokyo. From this our figure was taken. The largest specimen now known is in the United States National Museum and is fourteen feet in length. In the Upper Cretaceous is a very similar genus, Scapanorhynchus (lewisi, etc.), which Professor Woodward thinks may be even generically identical with Mitsukurina, though there is considerable difference in the form of the still longer rostral plate, and the species of Scapanorhynchus differ among themselves in this regard.
Fig. 327.—Goblin-shark (Tenguzame), Mitsukurina owstoni Jordan. From a young specimen in the Imperial University of Tokyo.
Mitsukurina, with Heterodontus, Heptranchias, and Chlamydoselache, is a very remarkable survival of a very ancient form. It is an interesting fact that the center of abundance of all these relics of ancient life is in the Black Current, or Gulf Stream, of Japan.
Fig. 328.—Scapanorhynchus lewisi Davis. Family Mitsukurinidæ. Under side of snout. (After Woodward.)
Family Alopiidæ, or Thresher Sharks.—The related family of Alopiidæ contains probably but one recent species, the great fox-shark, or thresher, found in all warm seas. In this species, Alopias vulpes, the tail is as long as the rest of the body and bent upward from the base. The snout is very short, and the teeth are small and close-set. The species reaches a length of about twenty-five feet. It is not especially ferocious, and the current stories of its attacks on whales probably arise from a mistake of the observers, who have taken the great killer, Orca, for a shark. The killer is a mammal, allied to the porpoise. It attacks the whale with great ferocity, clinging to its flesh by its strong teeth. The whale rolls over and over, throwing the killer into the air, and sailors report it as a thresher. As a matter of fact the thresher very rarely if ever attacks any animal except small fish. It is said to use its tail in rounding up and destroying schools of herring and sardines. Fossil teeth of thresher-sharks of some species are found from the Miocene.
Family Pseudotriakidæ.—The Pseudotriakidæ consist of two species. One of these is Pseudotriakis microdon, a large shark with a long low tail, long and low dorsal fin, and small teeth. It has been only twice taken, off Portugal and off Long Island. The other, the mute shark, Pseudotriakis acrales, a large shark with the body as soft as a rag, is in the museum of Stanford University, having been taken by Mr. Owston off Misaki.
Family Lamnidæ.—To the family of Lamnidæ proper belong the swiftest, strongest, and most voracious of all sharks. The chief distinction lies in the lunate tail, which has a keel on either side at base, as in the mackerels. This form is especially favorable for swift swimming, and it has been independently developed in the mackerel-sharks, as in the mackerels, in the interest of speed in movement.
Fig. 329.—Tooth of Lamna cuspidata Agassiz. Oligocene. Family Lamnidæ. (After Nicholson.)
The porbeagle, Lamna cornubica, known as salmon-shark in Alaska, has long been noted for its murderous voracity. About Kadiak Island it destroys schools of salmon, and along the coasts of Japan, and especially of Europe and across to New England, it makes its evil presence felt among the fishermen. Numerous fossil species of Lamna occur, known by the long knife-like flexuous teeth, each having one or two small cusps at its base.
Fig. 330.—Mackerel-shark, Isuropsis dekayi Gill. Pensacola, Fla.
In the closely related genus, Isurus, the mackerel-sharks, this cusp is wanting, while in Isuropsis the dorsal fin is set farther back. In each of these genera the species reach a length of 20 to 25 feet. Each is strong, swift, and voracious. Isurus oxyrhynchus occurs in the Mediterranean, Isuropsis dekayi, in the Gulf of Mexico, and Isuropsis glauca, from Hawaii and Japan westward to the Red Sea.
Man-eating Sharks.—Equally swift and vastly stronger than these mackerel-sharks is the man-eater, or great white shark, Carcharodon carcharias. This shark, found occasionally in all warm seas, reaches a length of over thirty feet and has been known to devour men. According to Linnæus, it is the animal which swallowed the prophet Jonah. "Jonam Prophetum," he observes, "ut veteris Herculem trinoctem, in hujus ventriculo tridui spateo bæsisse, verosimile est."
Fig. 331.—Tooth of Isurus hastalis (Agassiz). Miocene. Family Lamnidæ. (After Nicholson.)
It is beyond comparison the most voracious of fish-like animals. Near Soquel, California, the writer obtained a specimen in 1880, with a young sea-lion (Zalophus) in its stomach. It has been taken on the coasts of Europe, New England, Carolina, California, Hawaii, and Japan, its distribution evidently girdling the globe. The genus Carcharodon is known at once by its broad, evenly triangular, knife-like teeth, with finely serrated edges, and without notch or cusp of any kind. But one species is now living. Fossil teeth are found from the Eocene. One of these, Carcharodon megalodon (Fig. 332), from fish-guano deposits in South Carolina and elsewhere, has teeth nearly six inches long. The animal could not have been less than ninety feet in length. These huge sharks can be but recently extinct, as their teeth have been dredged from the sea-bottom by the Challenger in the mid-Pacific.
Fossil teeth of Lamna and Isurus as well as of Carcharodon are found in great abundance in Cretaceous and Tertiary rocks. Among the earlier species are forms which connect these genera very closely.
The fossil genus Otodus must belong to the Lamnidæ. Its massive teeth with entire edges and blunt cusps at base are common in Cretaceous and Tertiary deposits. The teeth are formed much as in Lamna, but are blunter, heavier, and much less effective as instruments of destruction. The extinct genus Corax is also placed here by Woodward.
Fig. 332.—Carcharodon megalodon Charlesworth. Miocene. Family Lamnidæ. (After Zittel.)
Family Cetorhinidæ, or Basking Sharks.—The largest of all living sharks is the great basking shark (Cetorhinus maximus), constituting the family of Cetorhinidæ. This is the largest of all fishes, reaching a length of thirty-six feet and an enormous weight. It is a dull and sluggish animal of the northern seas, almost as inert as a sawlog, often floating slowly southward in pairs in the spring and caught occasionally by whalers for its liver. When caught, its huge flabby head spreads out wide on the ground, its weight in connection with the great size of the mouth-cavity rendering it shapeless. Although so clumsy and without spirit, it is said that a blow with its tail will crush an ordinary whaleboat. The basking shark is known on all northern coasts, but has most frequently been taken in the North Sea, and about Monterey Bay in California. From this locality specimens have been sent to the chief museums of Europe. In its external characters the basking shark has much in common with the man-eater. Its body is, however, relatively clumsy forward; its fins are lower, and its gill-openings are much broader, almost meeting under the throat. The great difference lies in the teeth, which in Cetorhinus are very small and weak, about 200 in each row. The basking shark, also called elephant-shark and bone-shark, does not pursue its prey, but feeds on small creatures to be taken without effort. Fossil teeth of Cetorhinus have been found from the Cretaceous, as also fossil gill-rakers, structures which in this shark are so long as to suggest whalebone.
Fig. 333.—Basking Shark, Cetorhinus maximus (Gunner). France.
Family Rhineodontidæ.—The whale-sharks, Rhineodontidæ, are likewise sluggish monsters with feeble teeth and keeled tails. From Cetorhinus they differ mainly in having the last gill-opening above the pectorals. There is probably but one species, Rhineodon typicus, of the tropical Pacific, straying northward to Florida, Lower California, and Japan.
The Carcharioid Sharks, or Requins.—The largest family of recent sharks is that of Carchariidæ (often called Galeorhinidæ, or Galeidæ), a modern offshoot from the Lamnoid type, and especially characterized by the presence of a third eyelid, the nictitating membrane, which can be drawn across the eye from below. The heterocercal tail has no keel; the end is bent upward; both dorsal fins are present, and the first is well in front of the ventral fins; the last gill-opening over the base of the pectoral, the head normally formed; these sharks are ovoviviparous, the young being hatched in a sort of uterus, with or without placental attachment.
Some of these sharks are small, blunt-toothed, and innocuous. Others reach a very large size and are surpassed in voracity only by the various Lamnidæ.
The genera Cynias and Mustelus, comprising the soft-mouthed or hound-sharks, have the teeth flat and paved, while well-developed spiracles are present. These small, harmless sharks abound on almost all coasts in warm regions, and are largely used as food by those who do not object to the harsh odor of shark's flesh. The best-known species is Cynias canis of the Atlantic. By a regular gradation of intermediate forms, through such genera as Rhinotriacis and Triakis with tricuspid teeth, we reach the large sharp-toothed members of this family. Galeus (or Galeorhinus) includes large sharks having spiracles, no pit at the root of the tail, and with large, coarsely serrated teeth. One species, the soup-fin shark (Galeus zyopterus), is found on the coast of California, where its fins are highly valued by the Chinese, selling at from one to two dollars for each set. The delicate fin-rays are the part used, these dissolving into a finely flavored gelatine. The liver of this and other species is used in making a coarse oil, like that taken from the dogfish. Other species of Galeus are found in other regions, Galeus galeus being known in England as tope, Galeus japonicus abounding in Japan.
Fig. 334.—Soup-fin Shark, Galeus zyopterus (Jordan & Gilbert). Monterey.
Galeocerdo differs mainly in having a pit at the root of the tail. Its species, large, voracious, and tiger-spotted, are found in warm seas and known as tiger-sharks (Galeocerdo maculatus in the Atlantic, Galeocerdo tigrinus in the Pacific).
The species of Carcharias (Carcharhinus of Blainville) lack the spiracles. These species are very numerous, voracious, armed with sharp teeth, broad or narrow, and finely serrated on both edges. Some of these sharks reach a length of thirty feet. They are very destructive to other fishes, and often to fishery apparatus as well. They are sometimes sought as food, more often for the oil in their livers, but, as a rule, they are rarely caught except as a measure for getting rid of them. Of the many species the best known is the broad-headed Carcharias lamia, or cub-shark, of the Atlantic. This the writer has taken with a great hook and chain from the wharves at Key West. These great sharks swim about harbors in the tropics, acting as scavengers and occasionally seizing arm or leg of those who venture within their reach. One species (Carcharias nicaraguensis) is found in Lake Nicaragua, the only fresh-water shark known, although some run up the brackish mouth of the Ganges and into Lake Pontchartrain. Carcharias japonicus abounds in Japan.
Fig. 335.—Cub-shark, Carcharias lamia Rafinesque. Florida.
A closely related genus is Prionace, its species Prionace glauca, the great blue shark, being slender and swift, with the dorsal farther back than in Carcharias. Of the remaining genera the most important is Scoliodon, small sharks with oblique teeth which have no serrature. One of these, Scoliodon terræ-novæ, is the common sharp-nosed shark of our Carolina coast. Fossil teeth representing nearly all of these genera are common in Tertiary rocks.
Probably allied to the Carchariidæ is the genus Corax, containing large extinct sharks of the Cretaceous with broadtriangular serrate teeth, very massive in substance, and without denticles. As only the teeth are known, the actual relations of the several species of Corax are not certainly known, and they may belong to the Lamnidæ.
Fig. 336.—Teeth of Corax pristodontus.
Family Sphyrnidæ, or Hammer-head Sharks.—The Sphyrnidæ, or hammer-headed sharks, are exactly like the Carchariidæ except that the sides of the head are produced, so as to give it the shape of a hammer or of a kidney, the eye being on the produced outer edge. The species are few, but mostly widely distributed; rather large, voracious sharks with small sharp teeth.
The true hammer-head, Sphyrna zygæna, Fig. 337, is common from the Mediterranean to Cape Cod, California, Hawaii, and Japan. The singular form of its head is one of the most extraordinary modifications shown among fishes. The bonnet-head (Sphyrna tiburo) has the head kidney-shaped or crescent-shaped. It is a smaller fish, but much the same in distribution and habits. Intermediate forms occur, so that with all the actual differences we must place the Sphyrnidæ all in one genus. Fossil hammer-heads occur in the Miocene, but their teeth are scarcely different from those of Carcharias. Sphyrna prisca, described by Agassiz, is the primeval species.
The Order of Tectospondyli.—The sharks and rays having no anal fin and with the calcareous lamellæ arranged in one or more rings around a central axis constitute a natural group to which, following Woodward, we may apply the name of Tectospondyli. The Cyclospondyli (Squalidæ, etc.) with one ring only of calcareous lamellæ may be included in this order, as also the rays, which have tectospondylous vertebræ and differ from the sharks as a group only in having the gill-openings relegated to the lower side by the expansion of the pectoral fins. The group of rays and Hasse's order of Cyclospondyli we may consider each as a suborder of Tectospondyli. The origin of this group is probably to be found in or near the Cestraciontes, as the strong dorsal spines of the Squalidæ resemble those of the Heterodontidæ.
Fig. 337.—Hammer-head Shark, Sphyrna zygæna L. Hindustan. (After Day.)
Suborder Cyclospondyli.—In this group the vertebræ have the calcareous lamellæ arranged in a single ring about the central axis. The anal fin, as in all the tectospondylous sharks and rays, is wanting. In all the asterospondylous sharks, as in the Ichthyotomi, Acanthodei, and Chimæras, this fin is present. It is present in almost all of the bony fishes. All the species have spiracles, and in all are two dorsal fins. None have the nictitating membrane, and in all the eggs are hatched internally. Within the group there is considerable variety of form and structure. As above stated, we have a perfect gradation among Tectospondyli from true sharks, with the gill-openings lateral, to rays, which have the gill-opening on the ventral side, the great expansion of the pectoral fins, a character of relatively recent acquisition, having crowded the gill-openings from their usual position.
Family Squalidæ.—The largest and most primitive family of Cyclospondyli is that of the Squalidæ, collectively known as dogfishes or skittle-dogs. In the Squalidæ each dorsal fin has a stout spine in front, the caudal is bent upward and not keeled, and the teeth are small and varied in form, usually not all alike in the same jaw.
Fig. 338.—Dogfish, Squalus acanthias L. Gloucester, Mass.
The genus Squalus includes the dogfishes, small, greedy sharks abundant in almost all cool seas and in some tropical waters. They are known by the stout spines in the dorsal fins and by their sharp, squarish cutting teeth. They are largely sought by fishermen for the oil in their livers, which is used to adulterate better oils. Sometimes 20,000 have been taken in one haul of the net. They are very destructive to herrings and other food-fishes. Usually the fishermen cut out the liver, throwing the shark overboard to die or to be cast on the beach. In northern Europe and New England Squalus acanthias is abundant. Squalus sucklii replaces it in the waters about Puget Sound, and Squalus mitsukurii in Japan and Hawaii. Still others are found in Chile and Australia. The species of Squalus live near shore and have the gray color usual among sharks. Allied forms perhaps hardly different from Squalus are found in the Cretaceous rocks and have been described as Centrophoroides. Other genera related to Squalus live in greater depths, from 100 to 600 fathoms, and these are violet-black. Some of the deep-water forms are the smallest of all sharks, scarcely exceeding a foot in length. Etmopterus spinax lives in the Mediterranean, and teeth of a similar species occur in the Italian Pliocene rocks. Etmopterus lucifer,[150] a deep-water species of Japan, has a brilliant luminous glandular area along the sides of the belly. Other small species of deeper waters belong to the genera Centrophorus, Centroscymnus, and Deania. In some of these species the scales are highly specialized, pedunculate, or having the form of serrated leaves. Some species are Arctic, the others are most abundant about Misaki in Japan and the Madeira Islands, two regions especially rich in semi-bathybial types. Allied to the Squalidæ is the small family of Oxynotidæ with short bodies and strong dorsal spine. Oxynotus centrina is found in the Mediterranean, and its teeth occur in the Miocene.
Fig. 339.—Etmopterus lucifer Jordan & Snyder. Misaki, Japan.
Family Dalatiidæ.—The Dalatiidæ, or scymnoid sharks, differ from the Squalidæ almost solely in the absence of dorsal spines. The smaller species belonging to Dalatias (Scymnorhinus, or Scymnus), Dalatias licha, etc., are very much like the dogfishes.
They are, however, nowhere very common. The teeth of Dalatias major exist in Miocene rocks. In the genus Somniosus the species are of very much greater size, Somniosus microcephalus attaining the length of about twenty-five feet. This species, known as the sleeper-shark or Greenland shark, lives in all cold seas and is an especial enemy of the whale, from which it bites large masses of flesh with a ferocity hardly to be expected from its clumsy appearance. From its habit of feeding on fish-offal, it is known in New England as "gurry-shark." Its small quadrate teeth are very much like those of the dogfish, their tips so turned aside as to form a cutting edge. The species is stout in form and sluggish in movement. It is taken for its liver in the north Atlantic on both coasts in Puget Sound and Bering Sea, and I have seen it in the markets of Tokyo. In Alaska it abounds about the salmon canneries feeding on the refuse.
Family Echinorhinidæ.—The bramble-sharks, Echinorhinidæ, differ in the posterior insertion of the very small dorsal fins, and in the presence of scattered round tubercles, like the thorns of a bramble instead of shagreen. The single species, Echinorhinus spinosus reaches a large size. It is rather scarce on the coasts of Europe, and was once taken on Cape Cod. The teeth of an extinct species, Echinorhinus richardi, are found in the Pliocene.
Fig. 340.—Brain of Monkfish, Squatina squatina L. (After Duméril.)
Suborder Rhinæ.—The suborder Rhinæ includes those sharks having the vertebræ tectospondylous, that is, with two or more series of calcified lamellæ, as on the rays. They are transitional forms, as near the rays as the sharks, although having the gill-openings rather lateral than inferior, the great pectoral fins being separated by a notch from the head.
The principal family is that of the angel-fishes, or monkfishes (Squatinidæ). In this group the body is depressed and flat like that of a ray. The greatly enlarged pectorals form a sort of shoulder in front alongside of the gill-openings, which has suggested the bend of the angel's wing. The dorsals are small and far back, the tail is slender with small fins, all these being characters shared by the rays. But one genus is now extant, widely diffused in warm seas. The species if really distinct are all very close to the European Squatina squatina. This is a moderate-sized shark of sluggish habit feeding on crabs and shells, which it crushes with its small, pointed, nail-shaped teeth. Numerous fossil species of Squatina are found from the Triassic and Cretaceous, Squatina alifera being the best known.
Fig. 341.—Saw-shark, Pristiophorus japonicus Günther. Specimen from Nagasaki.
Family Pristiophoridæ, or Saw-sharks.—Another highly aberrant family is that of the sawsharks, Pristiophoridæ. These are small sharks, much like the Dalatiidæ in appearance, but with the snout produced into a long flat blade, on either side of which is a row of rather small sharp enameled teeth. These teeth are smaller and sharper than in the sawfish (Pristis), and the whole animal is much smaller than its analogue among the rays. This saw must be an effective weapon among the schools of herring and anchovies on which the sawsharks feed. The true teeth are small, sharp, and close-set. The few species of sawsharks are marine, inhabiting the shores of eastern Asia and Australia. Pristiophorus japonicus is found rather sparsely along the shores of Japan. The vertebræ in this group are also tectospondylous. Both the Squatina and Pristiophorus represent a perfect transition from the sharks and rays. We regard them as sharks only because the gill-openings are on the side, not crowded downward to the under side of the body-disk. As fossil, Pristiophorus is known only from a few detached vertebræ found in Germany.
Suborder Batoidei, or Rays.—The suborder of Batoidei, Rajæ, or Hypotrema, including the skates and rays, is a direct modern offshoot from the ancestors of tectospondylous sharks, its characters all specialized in the direction of life on the bottom with a food of shells, crabs, and other creatures less active than fishes.
The single tangible distinctive character of the rays as a whole lies in the position of the gill-openings, which are directly below the disk and not on the side of the neck in all the sharks. This difference in position is produced by the anterior encroachment of the large pectoral fins, which are more or less attached to the side of the head. By this arrangement, which aids in giving the body the form of a flat disk, the gill-openings are limited and forced downward. In the Squatinidæ (angel-fishes) and the Pristiophoridæ (sawsharks) the gill-openings have an intermediate position, and these families might well be referred to the Batoidei, with which group they agree in the tectospondylous vertebræ.
Other characters of the rays, appearing progressively, are the widening of the disk, through the greater and greater development of the fins, the reduction of the tail, which in the more specialized forms becomes a long whip, the reduction, more and more posterior insertion, and the final loss of the dorsal fins, which are always without spine, the reduction of the teeth to a tessellated pavement, then finally to flat plates and the retention of the large spiracle. Through this spiracle the rays breathe while lying on the bottom, thus avoiding the danger of introducing sand into their gills, as would be done if they breathed through the mouth. In common with the cyclospondylous sharks, all the rays lack the anal fin. The rays rarely descend to great depths in the sea. The different members have varying relations, but the group most naturally divides into thick-tailed rays or skates (Sarcura) and whip-tailed rays or sting-rays (Masticura). The former are much nearer to the sharks and also appear earliest in geological times.
Pristididæ, or Sawfishes.—The sawfishes, Pristididæ, are long, shark-like rays of large size, having, like the sawsharks, the snout prolonged into a very long and strong flat blade, with a series of strong enameled teeth implanted in sockets along either side of it. These teeth are much larger and much less sharp than in the sawsharks, but they are certainly homologous with these, and the two groups must have a common descent, distinct from that of the other rays. Doubtless when taxonomy is a more refined art they will constitute a small suborder together. This character of enameled teeth on the snout would seem of more importance than the position of the gill-openings or even the flattening and expansion of the body. The true teeth in the sawfishes are blunt and close-set, pavement-like as befitting a ray. (See Fig. 152.)
Fig. 342.—Sawfish, Pristis pectinatus Latham. Pensacola, Fla.
The sawfishes are found chiefly in river-mouths of tropical America and West Africa: Pristis pectinatus in the West Indies; Pristis zephyreus in western Mexico; and Pristis pectinatus in the Senegal. They reach a length of ten to twenty feet, and with their saws they make great havoc among the schools of mullets and sardines on which they feed. The stories of their attacks on the whale are without foundation. The writer has never found any of the species in the open sea. They live chiefly in the brackish water of estuaries and river-mouths.
Fossil teeth of sawfishes occur in abundance in the Eocene. Still older are vertebræ from the Upper Cretaceous at Maestricht. In Propristis schweinfurthi the tooth-sockets are not yet calcified. In Sclerorhynchus atavus, from the Upper Cretaceous, the teeth are complex in form, with a "crimped" or stellate base and a sharp, backward-directed enameled crown.
Rhinobatidæ, or Guitar-fishes.—The Rhinobatidæ (guitar-fishes) are long-bodied, shovel-nosed rays, with strong tails; they are ovoviviparous, hatching the eggs within the body. The body, like that of the shark or sawfish, is covered with nearly uniform shagreen. The numerous species abound in all warm seas; they are olive-gray in color and feed on small animals of the seabottoms. The length of the snout differs considerably in different species, but in all the body is relatively long and strong. Most of the species belong to Rhinobatus. The best-known American species are Rhinobatus lentiginosus of Florida and Rhinobatus productus of California. The names guitar-fish, fiddler-fish, etc., refer to the form of the body. Numerous fossil species, allied to the recent forms, occur from the Jurassic. Species much like Rhinobatus occur in the Cretaceous and Eocene. Tamiobatis vetustus, lately described by Dr. Eastman from a skull found in the Devonian of eastern Kentucky, the oldest ray-like fish yet known, is doubtless the type of a distinct family, Tamiobatidæ. It is more likely a shark however than a ray, although the skull has a flattened ray-like form.
Fig. 343.—Guitar-fish, Rhinobatus lentiginosus Garman. Charleston, S. C.
Closely related to the Rhinobatidæ are the Rhinidæ (Rhamphobatidæ), a small family of large rays shaped like the guitar-fishes and found on the coast of Asia. Rhina ancylostoma extends northward to Japan.
In the extinct family of Astrodermidæ, allied to the Rhinobatidæ, the tail has two smooth spines and the skin is covered with tubercles. In Belemnobatis sismondæ the tubercles are conical; in Astrodermus platypterus they are stellate.
Rajidæ, or Skates.—The Rajidæ, skates, or rays, inhabit the colder waters of the globe and are represented by a large number of living species. In this family the tail is stout, with two-rayed dorsal fins and sometimes a caudal fin. The skin is variously armed with spines, there being always in the male two series of specialized spinous hooks on the outer edge of the pectoral fin. There is no serrated spine or "sting," and in all the species the eggs are laid in leathery cases, which are "wheelbarrow-shaped," with a projecting tube at each of the four angles. The size of this egg-case depends on the size of the species, ranging from three to about eight inches in length. In some species more than one egg is included in the same case.
Most of the species belong to the typical genus Raja, and these are especially numerous on the coasts of all northern regions, where they are largely used as food. The flesh, although rather coarse and not well flavored, can be improved by hot butter, and as "raie au beurre noir" is appreciated by the epicure. The rays of all have small rounded teeth, set in a close pavement.
Fig. 344.—Common Skate, Raja erinacea Mitchill. Woods Hole, Mass.
Some of the species, known on our coasts as "barn-door skates," reach a length of four or five feet. Among these are Raja lævis and Raja ocellata on our Atlantic coast, Raja binoculata in California, and Raja tengu in Japan. The small tobacco-box skate, brown with black spots, abundant on the New England coast, is Raja erinacea. The corresponding species in California is Raja inornata, and in Japan Raja kenojei. Numerous other species, Raja batis, clavata, circularis, fullonica, etc., occur on the coasts of Europe. Some species are variegated in color, with eye-like spots or jet-black marblings. Still others, living in deep waters, are jet-black with the body very soft and limp. For these Garman has proposed the generic name Malacorhinus, a name which may come into general use when the species are better known. In the deep seas rays are found even under the equator. In the south-temperate zone the species are mostly generically distinct, Psammobatis being a typical form, differing from Raja. Discobatus sinensis, common in China and Japan, is a shagreen-covered form, looking like a Rhinobatus. It is, however, a true ray, laying its eggs in egg-cases, and with the pectorals extending on the snout. Fossil Rajidæ, known by the teeth and bony tubercles, are found from the Cretaceous onward. They belong to Raja and to the extinct genera Dynatobatis, Oncobatis, and Acanthobatis. The genus Arthropterus (rileyi) from the Lias, known from a large pectoral fin, with distinct cylindrical-jointed rays, may have been one of the Rajidæ, or perhaps the type of a distinct family, Arthropteridæ.
Fig. 345.—Numbfish, Narcine brasiliensis Henle, showing electric cells. Pensacola, Fla.
Narcobatidæ, or Torpedoes.—The torpedoes, or electric rays (Narcobatidæ), are characterized by the soft, perfectly smooth skin, by the stout tail with rayed fins, and by the ovoviviparous habit, the eggs being hatched internally. In all the species is developed an elaborate electric organ, muscular in its origin and composed of many hexagonal cells, each filled with soft fluid. These cells are arranged under the skin about the back of the head and at the base of the pectoral fin, and are capable of benumbing an enemy by means of a severe electric shock. The exercise of this power soon exhausts the animal, and a certain amount of rest is essential to recovery.
The torpedoes, also known as crampfishes or numbfishes, are peculiarly soft to the touch and rather limp, the substance consisting largely of watery or fatty tissues. They are found in all warm seas. They are not often abundant, and as food they have not much value.
Perhaps the largest species is Tetronarce occidentalis, the crampfish of our Atlantic coast, black in color, and said sometimes to weigh 200 pounds. In California Tetronarce californica reaches a length of three feet and is very rarely taken, in warm sandy bays. Tetronarce nobiliana in Europe is much like these two American species. In the European species, Narcobatus torpedo, the spiracles are fringed and the animal is of smaller size. To Narcine belong the smaller numbfish, or "entemedor," of tropical America. These have the spiracles close behind the eyes, not at a distance as in Narcobatus and Tetronarce. Narcine brasiliensis is found throughout the West Indies, and Narcine entemedor in the Gulf of California. Astrape, a genus with but one dorsal fin, is common in southern Japan. Fossil Narcobatus and Astrape occur in the Eocene, one specimen of the former nearly five feet long. Vertebræ of Astrape occur in Prussia in the amber-beds.
Fig. 346.—Teeth of Janassa linguæformis Atthey. Carboniferous. Family Petalodontidæ. (After Nicholson.)
Petalodontidæ.—Near the Squatinidæ, between the sharks and the rays, Woodward places the large extinct family of Petalodontidæ, with coarsely paved teeth each of which is elongate with a central ridge and one or more strong roots at base. The best-known genera are Janassa and Petalodus, widely distributed in Carboniferous time. Janassa is a broad flat shark, or, perhaps, a skate, covered with smooth shagreen. The large pectoral fins are grown to the head; the rather large ventral fins are separated from them. The tail is small, and the fins, as in the rays, are without spines. The teeth bear some resemblance to those of Myliobatis. Janassa is found in the coal-measures of Europe and America, and other genera extend upward from the Subcarboniferous limestones, disappearing near the end of Carboniferous time. Petalodus is equally common, but known only from the teeth. Other widely distributed genera are Ctenoptychius and Polyrhizodus.
Fig. 347.—Polyrhizodus radicans Agassiz. Family Petalodontidæ. Carboniferous of Ireland. (After McCoy.)
These forms may be intermediate between the skates and the sting-rays. In dentition they resemble most the latter.
Similar to these is the extinct family of Pristodontidæ with one large tooth in each jaw, the one hollowed out to meet the other. It is supposed that but two teeth existed in life, but that is not certain. Nothing is known of the rest of the body in Pristodus, the only genus of the group.
Dasyatidæ, or Sting-rays.—In the section Masticura the tail is slender, mostly whip-like, without rayed dorsal or caudal fins, and it is usually armed with a very long spine with saw-teeth projecting backward. In the typical forms this is a very effective weapon, being wielded with great force and making a jagged wound which in man rarely heals without danger of blood-poisoning. There is no specific poison, but the slime and the loose cuticle of the spine serve to aggravate the irregular cut. I have seen one sting-ray thrust this spine through the body of another lying near it in a boat. Occasionally two or three of these spines are present. In the more specialized forms of sting-rays this spine loses its importance. It becomes very small and not functional, and is then occasionally or even generally absent in individuals.
The common sting-rays, those in which the caudal spine is most developed, belong to the family of Dasyatidæ. This group is characterized by the small skate-like teeth and by the non-extension of the pectoral rays on the head. The skin is smooth or more or less rough. These animals lie flat on the sandy bottoms in nearly all seas, feeding on crabs and shellfish. All hatch the eggs within the body. The genus Urolophus has a rounded disk, and a stout, short tail with a caudal fin. It has a strong spine, and for its size is the most dangerous of the sting-rays. Urolophus halleri, the California species, was named for a young man who was stung by the species at the time of its first discovery at San Diego in 1863. Urolophus jamaicensis abounds in the West Indies, Urolophus mundus at Panama, and Urolophus fuscus in Japan. None of the species reach Europe. The true sting-ray (stingaree, or clam-cracker), Dasyatis, is more widely diffused and the species are very closely related. In these species the body is angular and the tail whip-like. Some of the species reach a length of ten or twelve feet. None have any economic value, and all are disliked by fishermen. Dasyatis pastinaca is common in Europe, Dasyatis centrura along our Atlantic coast, Dasyatis sabina ascends the rivers of Florida, and Dasyatis dipterura abounds in the bay of San Diego. Other species are found in tropical America, while still others (Dasyatis akajei, kuhlii, zugei, etc.) swarm in Japan and across India to Zanzibar.
Fig. 348.—Sting-ray, Dasyatis sabina Le Sueur. Galveston.
Pteroplatea, the butterfly-ray, has the disk very much broader than long, and the trivial tail is very short, its little spine more often lost than present. Different species of this genus circle the globe: Pteroplatea maclura, on our Atlantic coast; Pteroplatea marmorata, in California; Pteroplatea japonica, in Japan; and Pteroplatea altavela, in Europe. They are all very much alike, olive, with the brown upper surface pleasingly mottled and spotted.
Sting-rays of various types, Tæniura, Urolophus, etc., occur as fossils from the Eocene onward. A complete skeleton called Xiphotrygon acutidens, distinguished from Dasyatis by its sharp teeth, is described by Cope from the Eocene of Twin Creek in Wyoming. Vertebræ of Urolophus are found in German Eocene. Cyclobatis (oligodactylus), allied to Urolophus, with a few long pectoral rays greatly produced, extending over the tail and forming a rayed wreath-like projection over the snout, is known from the Lower Cretaceous.
Myliobatidæ.—The eagle-rays, Myliobatidæ, have the pectoral fins extended to the snout, where they form a sort of rayed pad. The teeth are very large, flat, and laid in mosaic. The whip-like tail is much like that in the Dasyatidæ, but the spine is usually smaller. The eagle-like appearance is suggested by the form of the skull. The eyes are on the side of the head with heavy eyebrows above them. The species are destructive to clams and oysters, crushing them with their strong flat teeth.
In Aëtobatus the teeth are very large, forming but one row. The species Aëtobatus narinari is showily colored, brown with yellow spots, the body very angular, with long whip-like tail. It is found from Brazil to Hawaii and is rather common.
In Myliobatis the teeth are in several series. The species are many, and found in all warm seas. Myliobatis aquila is the eagle-ray of Europe, Myliobatis californicus is the batfish of California, and Myliobatis tobijei takes its place in Japan.
In Rhinoptera the snout is notched and cross-notched in front so that it appears as if ending in four lobes at the tip. These "cow-nosed rays," or "whipparees," root up the soft bottoms of shallow bays in their search for clams, much as a drove of hogs would do it. The common American species is Rhinopterus bonasus. Rhinoptera steindachneri lives in the Gulf of California.
Teeth and spines of all these genera are common as fossils from the Eocene onwards, as well as many of the extinct genus, Ptychodus, with cyclospondylous vertebræ. Ptychodus mammilaris, rugosus, and decurrens are characteristic of the Cretaceous of England. Myliobatis dixoni is common in the European Eocene, as is also Myliobatis toliapicus and Aëtobatis irregularis. Apocopodon seriacus is known from the Cretaceous of Brazil.
Fig. 349.—Eagle-ray, Aëtobatis narinari (Euphrasen). Cedar Keys, Fla.
Family Psammodontidæ.—The Psammodontidæ are known only from the teeth, large, flat, or rounded and finely dotted or roughened on the upper surface, as the name Psammodus (ψάμμος, sand; ὀδούς, tooth) would indicate. The way in which the jaws lie indicates that these teeth belonged to rays rather than sharks. Numerous species have been described, mostly from the Subcarboniferous limestones. Archæobatis gigas, perhaps, as its name would indicate, the primeval skate, is from the Subcarboniferous limestone of Greencastle, Indiana. Teeth of numerous species of Psammodus and Copodus are found in many rocks of Carboniferous age. Psammodus rugosus common in Carboniferous rocks of Europe.
Fig. 350.—Devil-ray or Sea-devil, Manta birostris (Walbaum). Florida.
Family Mobulidæ.—The sea-devils, Mobulidæ, are the mightiest of all the rays, characterized by the development of the anterior lobe of the pectorals as a pair of cephalic fins. These stand up like horns or cars on the upper part of the head. The teeth are small and flat, tubercular, and the whip-like tail is with or without spine. The species are few, little known, and inordinately large, reaching a width of more than twenty feet and a weight, according to Risso, of 1250 pounds. When harpooned it is said that they will drag a large boat with great swiftness. The manta, or sea-devil, of tropical America is Manta birostris. It is said to be much dreaded by the pearl-fishers, who fear that it will devour them "after enveloping them in its vast wings." It is not likely, however, that the manta devours anything larger than the pearl-oyster itself. Manta hamiltoni is a name given to a sea-devil of the Gulf of California. The European species Mobula edentula reaches a similarly enormous size, and Mobula hypostoma has been scantily described from Jamaica and Brazil. Mobula japonica occurs in Japan. A fœtus in my possession from a huge specimen taken at Misaki is nearly a foot across. In Mobula (Cephaloptera) there are teeth in both jaws, in Manta (Ceratoptera) in the lower jaw only. In Ceratobatis from Jamaica (C. robertsi) there are teeth in the upper jaw only. Otherwise the species of the three genera are much alike, and from their huge size are little known and rarely seen in collections. Of Mobulidæ no extinct species are known.