PISCES 463 



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 mem- 

 brane, and also adhering to the fin rays of Rhombus laevis. I 

 also found a species (M. dubium) in a cyst attached to the 

 ovary of Gasterosteus spinachii. Several species of amphisto- 

 matoid worms were found by Natterer in Brazilian fishes 

 (Cataphractus, &c.), some of these representing distinct genera 

 (Aspidocotylus, Notocotylus) , to which I found Sonsino's remark- 

 able fluke (Gastrodiscus Sonsinonis, mihi) from the horse to be 

 very closely allied. In this connection must also be men- 

 tioned Griibe and Wagener's curious Amphiptyches urna, found 

 attached to the branchiae, and also in the intestines of Chimaera 

 monstrosa. 



In addition to the above families and genera of digenetic flukes 

 infesting fishes we have the monogenetic tristomes and poly- 

 stomes. As remarked in my introductory treatise, the Tristo- 

 midas display a leech-like aspect, in consequence of which they 

 have been placed either along with the Malacobdellida, or in 

 some other allied family of the suctorial annelids. The tris- 

 tomes are not strictly entozoa, yet their internal organisation 

 conforms more to the Trematoda than to the Hirudinidce. 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 develop- 

 ment 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 vorti- 

 cells," On quitting the shell the embryonic Udonella at once 



