PART III (PISCES).

Swarms of entozoa infest fishes, and it is hard to say whether they are less numerous in the inhabitants of fresh water than in those of salt water. More attention has been paid to the helminths of the fishes than to the internal parasites of birds and reptiles, consequently, the number of known species must be estimated by many hundreds. In like manner a great deal has been written respecting the ectozoa of fishes. These parasites, often called fish-lice, belong chiefly to the haustellated crustaceans, and are better known by the title of _Epizoa_. No account of them can be afforded in this treatise, but some trifling notice of the literature of the subject will be given below.

About a hundred distinct species of fluke have been described as infesting fishes. Not many of these worms possess more than a zoological interest; nevertheless, from that point of view certain types are very curious. Most of the species dwell in the stomach and intestines, but, as more or less remarkable exceptions, I may mention _Distoma seriale_ infesting the kidney of _Salmo umbla_, _D. longum_ (Leidy) from the pharynx of _Esox estor_, _D. polymorphum_ from the urinary bladder of the common pike (_Esox lucius_), _D. obesum_ from the gall bladder of _Salminius_ and other Brazilian fishes (_Xiphostoma_, _Leporinus_), _D. tornatum_ attached to the gills of _Coryphæna hippuris_, _D. rosaceum_ attached to the palate of _Lota communis_, and _D. contortum_ attached to the gills of _Orthagoriscus mola_. Most of the forms found encysted are sexually-immature worms. To these belong _D. annuligerum_, found by Nordmann in cysts in the vitreous humour of the eye of the perch (_Perca fluviatilis_), and _D. embryo_ from the liver and peritoneum of _Acerina vulgaris_. One of the largest and most remarkable of the flukes inhabiting marine fishes is the _Distoma clavatum_, found by Tilesius in the stomach of _Pelamys_, by Pohl in _Thynnus_, and by Bosc in _Coryphæna_. In the last-named fish it has been found adhering to the gills, in the liver, and in the intestines. In August, 1865, I obtained this parasite from a sword-fish (_Xiphias gladius_), and in the same piscine host I also found examples of four other species of helminths (_Tetrarhynchus attenuatus_, scolex of another tetrarhynch, _Bothriocephalus plicatus_, _Ascaris incurva_). Believing _Distoma clavatum_ to represent several forms hitherto regarded as distinct, I append a few particulars respecting it. Five examples of this worm were obtained by me from the stomach of a sword-fish. Generally they varied in length from four lines to two inches. They differed somewhat in shape, but all had the so-called head and neck directed backwards. Below the ventral sucker the two largest specimens were distended with eggs and black pigment. All of them likewise exhibited more or less well-marked transverse rugæ, the last ring surrounding an orifice which represented the outlet of a large contractile vesicle. The eggs averaged 1/800″ in length.

When revising the entozoa of the Museum of the Royal College of Surgeons I encountered many parasites without labels attached. Amongst these were several flukes, which, though differing from each other in size and shape, appeared to be identical. One of these specimens turned out to be the particular _Distoma clavatum_ described and figured by Professor Owen in the ‘Zoological Society’s Transactions.’ Several of the others I made out to be part of a series contributed by Mr George Bennett, who also gave specimens to the British Museum, but the College Museum stores contained yet a third group of specimens of uncertain history. The large fluke described by Prof. Owen was formerly in the collection of the Rev. Lansdown Guilding. In Dr Baird’s catalogue the specimens presented by Mr Bennett are stated to have come from the stomach of a bonito, and probably Mr Guilding’s specimens may be referred to the same “host.” Be that as it may, the specimens differ from each other in a very striking manner. In the year 1730 M. Garsin first described this worm under the generic title of _Hirudinella_. He says:--“Cet insecte tiré de l’estomac de la Bonita ne vécut qu’environ deux heures. Exposé à l’air il étoit languissant, et reprenoit de la vivacité dans de l’eau de mer. Il diminua sensiblement de volume pendant qu’il vivoit encore.” M. Garsin’s description is accompanied by three figures. His specimens do not appear to have exceeded 1-1/2″ in length. In 1774 Pallas described a trematode (_Fasciola ventricosa_). It measured two inches in length. All that he says regarding its source is as follows:--“Ex Amboyna missum fuit singulare hoc molluscum, quod ad aliud quam Fasciolarum genus referre non potui, in quo quasi gigas erit.” He remarks upon its pale white color, and notices particularly the soft elastic body proper, which when wounded gave out a dark matter resembling soot. This material, when examined with the microscope, appeared fresh; it was not the result of decomposition. Pallas also gives many other details, accompanied by a figure. In 1790 Menzies likewise described and figured a fluke about two inches long. He calls it _Fasciola clavata_:--“It is of whitish color, somewhat pellucid, discharging at its mouth a black-colored fluid, which can easily be perceived through its body. I have often found it,” he adds, “in the maws of the bonito, between the tropics in the Pacific Ocean.” Notwithstanding the similarity of description, Menzies does not appear to have recognised the identity of his worm with that described by Pallas. Prof. Owen, however, subsequently established this identity, and referred to this species as the _Fasciola clavata_ seu _ventricosa_. On the other hand, the British Museum Catalogue represents Pallas’s worm as specifically distinct from that of Menzies, but as identical with the specimen described by Prof. Owen from Mr Guilding’s collection.

In 1802 Bosc described and figured a trematode under the title of _Fasciola fusca_. This he obtained from the intestines of a dorado. In form it differs considerably from the foregoing species. Bosc’s description runs as follows:--“Brune, la partie postérieure très-renflée, presque ovale, la partie antérieure mince, cylindrique, inégale, avec deux petits tentacules en dessous. Le suçoir de l’anus très grand.” Bosc recognised the identity of this worm with the _Distoma coryphænæ_ of Rudolphi, and systematists generally have adopted his synonymy. In the British Museum Catalogue the _Fasciola fusca_ and _F. ventricosa_ of Pallas are regarded as one and the same species. The existence of two small tentacles is certainly peculiar.

In 1827 Nardo obtained two very large flukes from the stomach of a fish captured in the Gulf of Venice during the month of September. He calls the fish _Prostostegus prototypus_, which appears to be the same as the _Luvarus imperialis_ of Rafinesque. One of the parasites, being five inches in length, he named _Distoma gigas_. His description is as follows:--“Distoma teres, rubrum, retractile; poro ventrali minimo cujus apertura magna, rotunda, ciliata; poro antico terminali, parvo; collo brevi, retrorsum divergente, extensili, apice angusto, basi lato; cauda longa, postice incrassata et in apice obtusa oscula donata.” The alleged ciliated character of the ventral sucker, was perhaps due to a wrinkled state of the lip. Apart from this character, I see no reason for supposing this parasite to be distinct from the _Distoma clavatum_ procured by Mr Guilding, or the _Fasciola ventricosa_ described by Pallas. The intestines of the fish harbored another parasite (_D. Raynerianum_). Unfortunately, Nardo gives no figure of _Distoma gigas_. It is the longest fluke known to science.

In the year 1835 Professor Owen communicated the memoirs already alluded to. In his paper he discussed questions relating to the structure of _Distoma clavatum_, and threw much light upon its anatomy, but I believe that the large “lateral cavities” described by Owen are neither more or less than the somewhat unusually distended alimentary cæca.

In 1845 Dujardin placed the worm with the true distomes, yet, at the same time, expressed grave doubts as to whether it were, in any sense, a fluke. “Ce ver,” he remarks, “n’est certainement pas un distome ni même un trématode. Si sa forme extérieure et ses deux oscules lui donnent quelque ressemblance avec les distomes, sa structure musculeuse la rapproche davantage des Gordius, et son tégument ressemble à celui des siponcles.” M. Dujardin examined the specimens preserved in the Paris Museum, and with regard to one particular example, described as “Fasciola, trouvé dans la mer de Nice,” he says, it presents “une certaine analogie avec le prétendu _Distoma clavatum_.” Dujardin himself was somewhat puzzled by the resemblance in question. He does not appear to have examined fresh specimens, yet he mentions the species as tolerably common in the bonito, and occasionally present in the tunny. At all events, it appears that the rightly so-called _Distoma clavatum_ is not unfrequently taken from the ocean in the free state. In concluding my notice of this remarkable worm I can only add that after examining numerous specimens both in the fresh and preserved states, I have formed the opinion that the following specific names all refer to one and the same parasite:--_Distoma clavatum_, Rudolphi; _D. coryphænæ_, Rud.; _D. gigas_, Nardo; _Fasciola clavata_, Menzies; _F. coryphænæ_, Bosc; _F. coryph. hippuridis_ and _F. Scombri pelamidis_, Tilesius; _F. fusca_, Bosc; _Hirudinella marina_, Garsin; _H. clavata_, Baird. In this list of synonyms we may probably also include Rudolphi’s _Distoma tornatum_.

In addition to these distomes there are numerous piscine flukes which may fairly be relegated to other genera. Thus, provisionally, I elevated Dujardin’s sub-genus _Echinostoma_ into a separate genus; and on what I considered sufficient grounds I established several other new genera from amongst the more curious flukes that had been described as infesting fishes (_Wedlia_, _Köllikeria_). In the genus _Echinostoma_ the oral sucker is either surrounded by a circle of little spines, or it occupies the centre of a disk, which is cleft at the ventral or anterior aspect. In the latter case the disk is either bordered both laterally and above by spines, or there are two large lobed appendages, whose margins are furnished with spines. In other respects this genus nearly corresponds with the distomes, the simple digestive tubes bifurcating immediately below the œsophageal bulb. The specimen of _Ech. hispidum_ here drawn (Fig. 77) was taken by me from the spiral intestine of a sturgeon (1855), in which fish it occurs very abundantly. The figure represents a back view of the head and a lateral view of the body, the neck having been slightly twisted. The ventral sucker is concealed, but the transparency of the skin permits a view of the internal organs.

Another remarkable genus, established by Von Siebold, is _Gasterostoma_. In this genus the ventral sucker has taken the position usually assigned to the oral opening; the latter being near the centre of the body. The digestive cæca also disappear, leaving only a short stomachal cavity, which reminds one of the same viscus in imperfectly organised sporocysts or rediæ. When _G. gracilescens_ first came under my observation I followed Rudolphi in describing it as a distome (_D. gracilescens_). The anatomy of the genus has been illustrated by Von Siebold; from whose observations also it may be inferred that the larvæ are various forms of _Bucephali_. Prof. Molin describes the water-vascular or respiratory apparatus as consisting (in _G. fimbriatum_) of a broad central tube, occupying the entire length of the body and opening externally at the tail.

Amongst the more remarkable fluke-types may be mentioned Van Beneden’s Nematobothrium (_N. filarina_), occupying the branchial cavity of _Sciæna aquila_, also _Holostoma clavus_, found by Molin in the intestines of _Gadus merlucius_, also _Köllikeria filicollis_, occupying open follicles in the branchial cavity of _Brama Raii_. The sexes in the last-named genus are distinct; male and female worms together occupying each cyst. The genus _Monostoma_ is also largely represented amongst fishes. Prof. Wedl found a species (_M. Wedlii_) occupying follicles in the intestinal mucous membrane, and also adhering to the fin rays of _Rhombus lævis_. I also found a species (_M. dubium_) in a cyst attached to the ovary of _Gasterosteus spinachii_. Several species of amphistomatoid worms were found by Natterer in Brazilian fishes (_Cataphractus_, &c.), some of these representing distinct genera (_Aspidocotylus_, _Notocotylus_), to which I found Sonsino’s remarkable fluke (_Gastrodiscus Sonsinonis_, mihi) from the horse to be very closely allied. In this connection must also be mentioned Grübe and Wagener’s curious _Amphiptyches urna_, found attached to the branchiæ, and also in the intestines of _Chimæra monstrosa_.

In addition to the above families and genera of digenetic flukes infesting fishes we have the monogenetic tristomes and polystomes. As remarked in my introductory treatise, the Tristomidæ display a leech-like aspect, in consequence of which they have been placed either along with the _Malacobdellidæ_, or in some other allied family of the suctorial annelids. The tristomes are not strictly entozoa, yet their internal organisation conforms more to the _Trematoda_ than to the _Hirudinidæ_. Thus, they support two small suckers anteriorly and one large sucker posteriorly, the body being externally smooth and devoid of annulations. The tristomes have therefore no anus. In some species the large caudal sucker is sessile, in others it is stalked or pedunculated, being in either case bordered by a membranous fold (Dujardin). All the species are hermaphroditic. They attach themselves to the gills of fishes or to the general surface, selecting especially the neighbourhood of the fins. Some species are parasitic on crustacean parasites that are themselves attached to marine fishes. In the genus _Udonella_ the mode of development is known to be simple and direct. According to Van Beneden, the embryos are large and acquire the form and characters of their parents whilst they are still within the egg-shell. They are ready to assume an independent existence the moment they quit the shell. The eggs are oval, the chorion being prolonged into a single filamentary process or “holdfast.” Van Beneden compares a group of them to a “bouquet of vorticells.” On quitting the shell the embryonic _Udonella_ at once attaches itself to the _Caligus_, and there acquires the adult condition. The _Polystomidæ_ comprise a variety of remarkable genera. I accept this family as the equivalent of Dujardin’s first group of trematodes which he termed “_Onchobothriens_,” rejecting only his genus _Diporpa_, which is a juvenile condition of _Diplozoon_. In this family Van Beneden includes the genera _Calceostoma_ and _Gyrodactylus_. In all the polystomes we have a more or less ramified intestine, but the reproductive organs conform to the general trematode type. All are hermaphroditic, the eggs being supplied with filamentary appendages, in some only at one pole of the shell, in others at both ends. The water-vascular system is conspicuously developed. All the species are supplied with prehensile hooks.

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.

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_).

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_.

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.

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.

From the large number of species of _Echinorhynchi_ infesting our fresh-water fishes, they present quite a feature of piscine parasitism. Almost every perch, chub, carp, pike, barbel, bream, or roach that one opens is found to have its intestines occupied by parasites which exhibit a light yellow color. These are Echinorhynchi, the common forms being _E. proteus_, _E. angustatus_ (Fig. 84, No. 1), _E. clavæceps_, _E. globulosus_, and _E. tuberosus_. In the Salmonidæ, besides several of the above, we may also find _E. clavula_, _E. fusiformis_, and _E. pachysomus_. As a group these parasites are more attractive looking than most other helminths, and they will well repay the zoological collector. The species infesting marine fishes are almost as numerous as those found in fresh-water hosts.

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