Fig. 79.—Gyrodactylus elegans, containing an embryo. a, a, Œsophagus; g, testis; h, h, sucker; i, i, large hooks; k, spines. Magnified. After Van Beneden.
In the Diporpa condition of Diplozoon there are two supernumerary hooks, associated with a dorsal sucker at the centre of the body, and it is by means of these organs that a conjugation between two such juvenile forms is effected. These two individuals become organically united for life, after the fashion of the Siamese twins. After conjugation the sexual organs appear. In Onchotyle appendiculata the lower end of the body merges into a curious appendage, which is placed almost at a right angle with the body itself, and in this way, as Van Beneden justly remarks, the entire animal resembles a little hammer, the resemblance being very much heightened by the circumstance that one end of the appendage is cleft so as to correspond, as it were, with the notch which we employ in the action of nail-drawing. The Onchotyle appendiculata was first discovered by Kuhn attached to the gills of a dog-fish (Scillium catulus), but it has since been found ectoparasitically lodged upon other marine fishes. With the Gyrodactylidæ I include Van Beneden’s genus Calceostoma. The gyrodactyles have been classed with the Polystomidæ. Amongst the characters standing out most prominently are those having reference to peculiar hooks which project from the great sucking disk. In Calceostoma this mechanism is reduced to a single horny structure placed at the margin of the caudal sucker in the central line. In some Gyrodactyli the hooks are very numerous. In Gyrodactylus elegans the caudal sucker supports a pair of large laterally-curved hooks, which are placed back to back in the centre of the disk, being connected at their upper ends by a supplementary semi-lunar bar. A series of tentacles serve to increase the prehensile action of the sucker. In many species the males are supplied with accessory horny developments. The genus Gyrodactylus has been studied by Nordmann, Von Siebold, G. Wagener, Van Beneden, and especially by Wedl, who records the following results:—(a.) “Gyrodactylus is found on the gills of fresh-water fishes under numerous specific forms, G. elegans being also found by Creplin and Siebold on the fins. Moreover, as I have found nearly every species of fish supporting a particular gyrodactyle representative, it would seem that each finny creature supplies its own Gyrodactylus. Sometimes two of them are parasitic upon the same gill, being frequently associated with Trichodinæ, as well as with the still unintelligible Psorospermiæ. (b.) The clasping apparatus at the posterior end of the body must—in an animal so soft and constantly exposed to the passage of regular currents—be comparatively strongly developed and accommodated to the peculiar dwelling-places, and probably the varying character of the latter supplies a reason why there should be so great a difference in the mechanism of the hooks belonging to the disk. (c.) The hooked apparatus affords a very valuable and mathematically precise means of diagnosis in the determination of species. This differentiation may be accomplished by observing whether there are two or four large hooks; whether there be one or two connecting portions, and by noticing their several forms and relations to one another; and whether, again, there are hooklets or not, remarking in the first instance their position, form, distribution, and so forth. (d.) The integument is sometimes wrinkled transversely, at other times appearing to be smooth. (e.) The muscular apparatus is, in certain cases, very strongly developed. In the majority of instances special muscles are inserted into the handles of the hooks, and they are also very frequently directed into the transverse muscles of the skin. In Gyrodactylus crassiusculus we find a protrusor penis and retractor palparum medius. (f.) Except in the case of G. elegans, four so-called eye-spots are observed at the anterior extremity of all Gyrodactyli. As Siebold says, they answer the purpose of light-refracting organs. The palpi, which in G. crassiusculus are seen to contain muscular bundles, appear to be retractile touch-organs, extending more or less prominently forward. (g.) Observations in regard to the alimentary canal are at present incomplete, for only in the case of G. cochlea did I find a single gullet demonstrable. (h.) Gyrodactylus becomes sexually developed, and cannot be regarded merely as a kind of ‘nurse.’”
So much for Wedl, whose views I have elsewhere recorded at great length. The genetic relations subsisting amongst the Gyrodactyles have given rise to much controversy. Observing the singular mode of reproduction in G. elegans, Von Siebold arrived at the conclusion that Gyrodactyles in general were only nurse-forms of some higher organism, and he pointed out, with undeniable accuracy, all the birth-stages of the young one as it apparently pullulated within the parent and subsequently emerged an almost perfect Gyrodactyle. Von Siebold also remarked that the so-called “daughter,” at the time of birth, nearly equalled the “parent” in respect of size, whilst, moreover, it contained within its interior another very young Gyrodactyle, or, in other words, a “grand-daughter.” Van Beneden interpreted these facts very differently. I have myself noticed the second generation, or daughter, to contain in its interior evidences of a third generation. This I observed in specimens obtained from the tails of Gasterostei caught in the Serpentine, Regent’s Park. Indications of the third progeny were seen whilst the daughter still resided within the body of the nurse-parent, and the so-called grand-daughter became much larger immediately after birth. In one instance the “daughter” commenced showing herself by a slight bulging at the centre of the parent’s body, whilst the integument of the latter yielded on all sides of the bud-like projection, and in such a manner as to convey the idea of a vaginal opening. There was an evident struggle on the part of the young one to free itself from the so-called parent envelope, but the tissues showed no signs of injury. On partial protrusion it was seen that the budding portion corresponded with the centre of the daughter’s body, and this, in a little while, assumed the aspect of a semicircular band. Subsequently the upper end became detached, the freed extremity being now recognised as the head. An interval elapsed before the broad posterior end of the animal could be disengaged, but immediately after this was effected the sides of the parent envelope closed in upon the opening, and all that remained was a small cavity or sac, indicating the position recently occupied by the daughter. Altogether the process occupied about five minutes. I carefully compared the so-called “parent” with the “daughter,” but in regard to size I can scarcely say which was the larger of the two. As before hinted, Van Beneden demurs altogether to Von Siebold’s views. He does not admit the parent to be a kind of “nurse,” he does not consider the primary young one to be a “daughter,” and, consequently, he does not regard the embryo seen within the latter as a “grand-daughter.” Van Beneden says:—“According to our researches there is here a false interpretation; the little daughter is lodged within the side of its pretended mother, and not in its interior; instead of being its mother, it is its sister; there is a difference of shape because there is a difference of age; the Gyrodactyles are viviparous, and as among the Trematodes the eggs are formed one by one, one embryo is scarcely formed when another commences its evolution, and the egg-deposition is effected even whilst the embryo is being produced. The Gyrodactyles are therefore viviparous worms, which beget a single embryo at a time, as those of the trematode group, to which they are allied, beget a single egg at a time, and before the first embryo is expelled another is already partly developed. There, we believe, lies the correct interpretation of that phenomenon; instead of a bud it is an embryo, which has escaped from an egg. Here, therefore, we have no phenomenon of alternate generation or of digenesis, as Von Siebold supposes, but a simple viviparous reproduction.”
Passing on to notice the cestodes of fishes, I may remark that they often display characters very distinctive from those inhabiting birds and mammals, being commonly furnished with special tentacular hook-appendages employed as supplementary organs of boring and anchorage. In the cartilaginous sharks and rays these cestodes are remarkably abundant, and in certain osseous species they are scarcely less frequent. The only noteworthy kinds of fish which are commonly free from the invasion of tapeworms are the sturgeons, blennies, gobios, mullets, sparoids, and Sciænæ. Some few of them are infested by Ligulæ, Caryophyllæi, &c. Cuttle fishes harbor a great variety of tapeworm-larvæ, forming one of the chief sources whence sharks and rays obtain the same parasites destined to arrive at sexual maturity within their own bodies.
Fig. 80.—Section of the strobile of Bothriocephalus proboscideus. Magnified. After Busk.
Among the most interesting cestodes of fishes we may reckon the pit-headed tapeworms and their allies (Bothriocephalidæ). One of the most common species is Both. proboscideus which is found, often in considerable numbers, lodged within the pyloric appendages of the salmon (Salmo salar and S. hucho). It acquires a length of two feet. When in large numbers it cannot fail to prove injurious to the bearer. In this connection also must be mentioned B. nodosus. In the adult state this worm infests a great variety of water-birds (herons, gulls, and divers), but in the young or sexually-immature tænioid condition it is a frequent inhabitant of sticklebacks (Gastereosteus aculeatus and G. pungitus), being also found in the salmon and in the bull-head, or father-lasher (Cottus scorpio). The immature tapeworm was formerly considered a separate species (B. solidus). Some years back Creplin discovered the connection subsisting between the two forms, and re-described the species in its two conditions under the name of Schistocephalus dimorphus, but it was reserved for Von Siebold to explain the full nature of this relationship. In his essay on “Tape and Cystic Worms” he shows that it is not until the worm reaches the intestine of the ultimate host that its segments acquire sexual completeness. As Von Siebold observes, “the extent of development in each individual will be found to be in proportion to the time the parasite has passed in the bird’s alimentary canal after its passive immigration.” A similar instance, it is added, “occurs in the case of the Ligula simplicissima, infesting the abdominal cavity of various species of carp, whose sexual organs are, and remain, undeveloped as long as the worm resides within the fish; whilst, when the latter is eaten by ducks, divers, waders, and other water-fowl, the entozoon being thus conveyed into their intestine, it attains perfect sexual development. In the older helminthological works the sexually-mature Ligula simplicissima is described under various specific names (L. sparsa, L. uniserialis, L. alternans, L. interrupta).” These results have been confirmed by later observers, but it is now usual to recognise the sexually-mature worm as the Ligula monogramma of Creplin. In 1876 Dr Duchamp published his beautiful memoir on this subject, treating the entire question exhaustively and adding important experimental details. M. Duchamp gives a list of about twenty species of fish that are infested by the immature worm, and amongst these the Cyprinidæ play by far the most conspicuous part. M. Duchamp has recorded a fatal piscine epizoöty amongst tenches (Tinca vulgaris), occurring in the ponds of La Bresse. This is produced by Ligula simplicissima, which escapes by an aperture formed near the vent of the infested fish. M. Duchamp also gives important anatomical and embryological details, but the especially interesting part of his memoir refers to his feeding experiments, seven in number. He succeeded in rearing L. monogramma in the domestic duck, by feeding this bird with examples of L. simplicissima obtained from the abdomen of the tench (Tinca vulgaris). The interest of these experiments does not cease here, since they afford a probable clue to the source of human Bothriocephali, which in nearly all essential points of structure correspond with the Ligules. As remarked in the first part of this work, Leuckart long ago pointed to the Salmonidæ as probably furnishing the intermediate host of this worm; and he disproved the views of Knoch, of Petersburg, who thought he had reared Bothriocephalus latus in the dog in a direct manner. I have already called attention to the opinion of Dr Fock, of Utrecht, who thinks the human bearer may become infested by the consumption of the little fresh-water bleak (Leuciscus alburnus). From the observations of Dr Bertolus, it is extremely probable that our Bothriocephalus latus is the sexually-mature condition of Ligula nodosa infesting the abdominal cavity and pyloric appendages of the common trout (Salmo trutta).
Fig. 81.—Portion of the proboscis of a scolex of Tetrarhynchus infesting Merlangus vulgaris. Magnified. After Busk.
Another cestode of general interest is the Tricuspidaria (Triænophorus) nodulosus, infesting many of our fresh-water fishes. It varies in length from one to two feet. The segmentation of the strobila is very indistinct, but the reproductive organs occur at regular intervals. All parts of the body are extremely contractile, especially the head. The tricuspid hooks support thin chitinous laminæ, which connect the two lateral horns of each hook to the central apophysis. The object of this arrangement is to afford additional security to the prong-like processes. Van Beneden appears to think it an error that the cusps of the hooks should have been figured in ‘Règne Animal’ as directed forwards, and he has drawn the hooks with the points downwards. In regard to the calcareous corpuscles, narrow vessels may be easily recognised passing off continuously from the capsules in closing the particles. These vascular prolongations are single, having their course directed towards the epidermis; doubtless they open at the surface, but I did not detect any aperture. I have figured the tubes in my ‘Entozoa’ (p. 132). Dr Guido Wagener figures similar structures as occurring in Cercaria macrocerca.