Various species of Tetrarhynchus dwell in the bodies of sharks and rays, whilst their larvæ inhabit fishes on which the plagiostomi feed. Immature tetrarhynchs occur in cuttle-fishes, but they are most abundant in such fish as the cod, haddock, turbot, whiting (Fig. 81), flounder, sole, gurnard, mackerel, mullet, and conger-eel. A tænioid scolex constantly infests the muscles and viscera of the great sunfish. The tetrarhynchs differ from one another as regards the form of their proboscides and the relative number and disposition of the hooks. I must refer to my ‘Entozoa’ for a full description, with figures, of a larval tetrarhynch from the wall of the intestine of a haddock. Some Tetrarhynchi exhibit a very complex armature, as may be seen in Tetrarhynchus longicollis infesting the tope or penny dog-fish (Galeus vulgaris). In this species the hooks are uniform in size, and arranged in spirally disposed circles carrying from twenty to thirty hooks each. In the tetrarhynch from the whiting the hooks show much irregularity both as regards size and arrangement. A remarkable scolex infests the sun-fish (Orthagoriscus mola); it is a true tetrarhynch, but has been variously classed. According to view all the following titles refer to this parasite:—Gymnorhynchus reptans, Rudolphi; G. horridus, John Goodsir; Acanthorhynchus reptans, Diesing; Bothriorhynchus continuus, Van Lidth de Jeude; Bothriocephalus patulus, Leuckart; Acanthocephalus elongatus, Rudolphi; A. macrourus, Bremser; Floriceps saccatus, Cuvier; F. elongatus, Blainville; Scolex gigas, Cuvier; Tetrarhynchus reptans, Cobbold.
Fig. 82.—Tetrarhynchus reptans. 1, Reduced figure of a sunfish, showing the worms in sitû; 2, head of a worm in its capsule; 3, tænioid scolex; 4, section of the immature strobile, 5, proboscis; 6, row of hooks; 7, 8, large and small hooks (magnified 260 diameters); 9, head of the scolex viewed from above. Original.
Five or six examples of the sunfish have been examined by me in the fresh state, all of them being infested by tetrarhynchs. In the fish here drawn (fig. [82]) the liver and lateral muscles were extensively tunnelled by the parasite. In all instances the anterior part of the worm was found surrounded by a thick, clear, transparent cyst, which gradually diminished in thickness towards the tail. When liberated from its investing capsule the head of the worm presents a quadrilateral figure, each lateral half being furnished with a bipartite facet. The retractile boring organs are club-shaped, each supporting about 1600 hooks. Nearly all the hooks display a uniform length and thickness, but at the lower part of each proboscis there are two conspicuous circles, the hooks of which are at least twice as large as the others. The joints of the immature strobile are well formed, but exhibit no trace of sexual organs. If it be asked “what is the object of this perpetual tunnelling,” and “does the boring cause suffering to the host,” I reply:—“The object of tunnelling is apparently twofold; first, that the parasite may constantly obtain fresh nourishment; and secondly, that it may acquire another residence.” It furnishes an example of a parasite perpetually striving to perform an act which it cannot accomplish; for, in order to arrive at sexual maturity, it must wait until the sunfish is devoured by a shark. In regard to the question as to the boring action giving rise to pain, one cannot, of course, speak with absolute certainty. When there are many parasites occupying the liver, or other important viscera, then, doubtless, they create pain, and cause decay of the organs infested; thus they enfeeble the vital powers of the host. At such a time the sunfish would be easily overcome by its natural enemies, and be the first to succumb in the struggle for existence. These wandering tetrarhynchoid scolices never escape the body of the intermediate host until they are passively transferred into the alimentary canal of the ultimate entertainer. In the sharks and rays they acquire sexual maturity. From these animals the proglottides pass into the water in the ordinary way. The ova are subsequently swallowed by sunfishes and other intermediate hosts, within whose stomachs the six-hooked embryos are liberated, and the scolices become developed in the ordinary manner. As obtains in Cysticercus fasciolaris of the mouse the scolex of Tetr. reptans becomes tænioid. I have seen the liver of an adult sunfish so infested by these parasites that the whole organ might be fitly described as a mere bag of worms, the immature strobiles being inextricably coiled together and defying separation. One of the parasites which I removed from this particular fish is preserved in the Hunterian Collection.
In reference to the nematoids of fishes I can say but little. They are excessively abundant; sexually-immature filariæ being found in almost every marine fish that one examines. Even at our dinner and breakfast tables nothing is more common than to observe the little Filaria piscium spirally coiled within the tissues of herrings, haddocks, cod-fish, and whiting. All the sexually-immature nematoids are, as it were, waiting to be passively transferred to their ultimate hosts. These final bearers are usually either fishes, birds, cetacea, or seals. Amongst fresh-water fishes the Cucullanidæ play an important rôle. These parasites closely resemble the strongyloid Sclerostomata, but the absence of a true bursa seems to justify their separation into a distinct family. In most of them the body is truncated in front and much narrowed or drawn out posteriorly. The head is, broad and globular, and furnished with a powerful muscular pharynx. The mouth is seldom round; it is often subterminal, opening by a transverse slit. The tail of the male is recurved, and usually supplied with membranous winged appendages; sometimes there is a pre-anal sucking disk. In the female the tail is simple, and more or less sharply pointed.
The facts relating to the development of these parasites are especially interesting as having afforded Leuckart and Fedschenko a clue to what obtains in the guinea-worm (Dracunculus). The Cucullanus of the perch (C. elegans) is a viviparous species. The embryos are supplied with little boring teeth, or styles, which enable them to perforate the bodies of entomostracous crustaceans. Having in a direct manner gained access to the perivisceral cavity of Cyclops, they remain coiled within the intermediate bearer until it has been pursued, captured, and transferred to the stomach of the ultimate or piscine host. Once liberated within the stomach of the fish the young Cucullani soon acquire sexual maturity.
The acanthocephalous Echinorhynchi are very abundant in fishes. They also, like the Cucullani, require a change of hosts in order to ensure the continuance of the species. No less than six species of Echinorhynchi are known to infest the trout (Salmo fario). As many as four species likewise infest the eel (Anguilla); the same number of distinct forms being also found in the turbot (Rhombus) and ling (Lota), whilst three species may be met with in the common sole (Solea). What we at present know respecting the mode of development of Echinorhynchi infesting fishes is principally due to the researches of Leuckart. Some years back Dr Guido Wagener supplied admirable illustrations of the eggs and embryos of Echinorhynchi, but he was erroneously led to conclude that the larvæ were developed in a direct manner. The notion of a simple metamorphosis was entirely disproved by the experiments of Leuckart, who found the growth and development of the young to be accompanied by a true alternate generation. He showed this to obtain in Echinorhynchus proteus, a species abundant in the trout and in many other fresh-water fishes. The embryo of this parasite is broad and obliquely truncated at the ventral surface anteriorly, being gradually narrowed to a blunt point posteriorly, and at the front part, on each side of the middle line, there are five or six spines biserially disposed. Similar characters are seen in E. filicollis. Prof. Leuckart introduced a number of eggs into a vessel of water containing several small crustaceans (Gammarus Pulex). These little animals readily swallowed the ova, and in a few days the embryos were found emerging from their shells, boring their way through the intestinal walls, then passing into the general cavity of the body, and even into the appendages themselves. During the next fourteen days the embryos within the Gammari exhibited an increase of size; and in course of the third week a further metamorphosis caused the embryos to assume the readily recognisable characters of a young Echinorhynchus. Thus, in Leuckart’s own words, “the ultimate animal arises in the interior of the primordial body, by a process which presents so close an analogy with the production of an embryo, and, consequently, with the act of generation, that one feels inclined at once to identify it with such an act, and therefore, also, to regard the Echinorhynchus as exhibiting an alternation of generation in its mode of development rather than a metamorphosis.”
The young Echinorhynchus afterwards grows rapidly, its several internal organs, proboscideal sac, and muscular apparatus, gradually coming into view. At last the young entozoon completely fills the interior of the embryo, the latter having scarcely undergone any change, and still remaining, of course, within its crustacean host. What may be regarded as even more extraordinary is the circumstance that the embryonic body next becomes firmly adherent to the young Echinorhynchus, thus ultimately forming the true integument of the adult Echinorhynchus. The original skin of the embryo, however, is cast off “as soon as the Echinorhynchus occupies the whole interior of the embryo.” After this the sexual differences become clearly established. Leuckart remarks that the passage of the young Echinorhynchi into their ultimate host is probably unattended by any striking changes, whilst the metamorphosis of the embryo, as thus far detailed, occupies a period of about six weeks. In general the crustacean hosts appear to suffer little from the borings of the embryo parasites, but when the latter have assumed the Echinorhynchus-condition and happen to be particularly numerous they not unfrequently prove fatal to the unsuspecting Gammari. After their transference to the intestine of the ultimate host a period of about one week more is required for the completion of their development.