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.