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.
| Fig. 77.—Echinostoma hispidum. Magnified. Original. | Fig. 78.—Gasterostoma gracilescens. Magnified. Original. |
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.
