Habits and Structure of Ectoparasitic Trematodes.
I. Temnocephalidae.—These interesting forms, of which a good account has lately been written by Haswell,[[71]] occur on the surface (rarely in the branchial chamber) of fresh-water crayfish and crabs in Australasia, the Malay Archipelago, Madagascar, and Chili. Others have been found on the carapace of a fresh-water tortoise, and in the branchial chamber of the mollusc Ampullaria from Brazil. Wood-Mason discovered others, again, in bottles containing spirit-specimens of Indian fish. Temnocephala is rarely more than a quarter of an inch long, and looks like a minute Cephalopod or a broad flattened Hydra. By the ventral sucker each species adheres to its own particular host, the tentacles being used as an anterior sucker for "looping" movements. The food, consisting of Entomostraca, Rotifera, and Diatoms, is first swallowed whole by the large pharynx (Fig. 20, ph), which can be protruded through the ventrally-placed mouth, and is then received into a simple lobed intestine (d). The skin, especially on the surface of the tentacles, is provided here and there with patches of cilia borne by the cellular epidermis,—the only undoubted case of external cilia occurring in an adult Trematode. Minute rhabdites formed in special gland-cells, occur plentifully on the tentacles, and are another distinctly Turbellarian feature. The excretory system is peculiar (Fig. 21). Fine ducts proceed from the various organs of the body, and open to the exterior by means of a pair of contractile sacs placed on the dorsal surface. Each sac is a single cell, and within it not one merely, but several "flames," or bunches of rhythmically contractile cilia, are present. These are placed on the course of excessively fine canals, which perforate the protoplasm of this cell. The terminal branches of the excretory canals end in branched cells, apparently devoid of "flames."
Fig. 20.—Temnocephala novae-zealandiae Has. × 10. Ventral view to show the digestive and reproductive systems. (After Haswell.)
Fig. 21.—The same from the dorsal surface, to show the excretory system (double line), and the nervous system (black and shaded). (After Haswell.)
d, Intestine; dln, dorso-lateral nerve; dn, dorsal nerve; ex.o, excretory aperture on dorsal surface; ex.s, terminal excretory sac; m, mouth; ov, ovary; ovd, oviduct; ph, pharynx; rh, rhabdites; rh.c, cells in which the rhabdites are formed; rv, yolk receptacle; sc, sucker; sh, shell-gland; te, testes; ut, uterus; vg, vagina; vn, ventral nerve; vs, vesicula seminalis; yd, yolk-duct; yg, yolk-gland. ♀, ♂, common genital pore.
The reproductive system is very similar to that of certain Rhabdocoels. An armed penis and the female genital duct open into a genital atrium, and this by a single aperture (♀, ♂, Fig. 20) to the exterior. The fertilised ovum and yolk are enclosed in a stalked shell formed in the uterus.
The interest and importance of the Temnocephalidae lies in the fact that they are almost as much Turbellaria as Trematodes. In habits, in the character of the skin, the muscular, digestive, and reproductive systems, they find their nearest allies in Rhabdocoels (Vorticidae). But in the excretory and nervous systems, the latter composed of two dorsal, two lateral, and two ventral trunks all connected together (Fig. 21), they are Tristomid Trematodes. Thus they may fitly connect an account of the two great groups.
Fig. 22.—A, Nematobothrium filarina van Bened. Nat. size. Two individuals (a and b) are found together, encysted on the branchial chamber of the Tunny. B, Udonella caligorum Johns. A Tristomid, several of which are attached to the ovary of a Copepod (Caligus), itself a parasite on the gills of the Hake. × 8. C, Epibdella hippoglossi O. F. M. A Tristomid found on the body of the Halibut. Nat. size. m, Mouth; ms, lateral suckers; ov, ovary; ps, posterior sucker; te, testes. (All after P. J. van Beneden.)