THE ENEMIES OF FISH. 513 



the D. ferniginosum of the barbel, the D. macrohothrium and tectum of 

 the smelt, the D. laiireatum and varicum of the "Aesche", the I>. ocrea- 

 tum of the herring, salmon, and May-fish, which, when young, lives 

 quite free, and is a parasite on larvse of worms and small crustaceans. 



Of the Polystonia I will only mention the Octohothrium of the May- 

 fish, which lives in the gills like the Diplozoon of the cyprinoids, the 

 Gyrodactylus and Dactylogyrus. 



The Acanthocephala are round, tube-formed worms, without mouth and 

 intestinal tube, whilst at the head-end they have a trunk with hooks. 

 Their embryos have smaller hooks, with which they pierce the intestinal 

 tube of the animals in which they live, principally crustaceans, then 

 wrap themselves \\\) in a capsule, and in that state are transferred to 

 other animals, fish, birds, or whales. 



We thus find the Ecliinorhyncus proteus when young in small crusta- 

 ceans, and when more developed in the perch, the " Wels," the carj), the 

 salmonoids, and other fish. It is likewise found in many salt-water fish, 

 as the codfish and the plaice; whilst it does not occur in the rays and 

 sharks.* 



Different from the Acanthocephala is the family of the Nematodes, num- 

 bering upwards of 1,200 kinds, distinguished chiefly by a more or less 

 developed organ of digestion. These Nematodes are very prolific, and 

 exhibit many peculiar phases of development ; a change of generation 

 does not, however, seem to occur with them, at any rate it has so far not 

 been observed. It has been noticed, however, that parasitical hermaph- 

 rodites have been produced by free individuals of opposite sexes. There 

 may also possibly be a development of the Qgg without impregnation. 



The Nematodes are very frequent in fish, sometimes half developed in 

 capsules, and sometimes fully grown. 



We thus find the Oordhis aquaticus, a very long (1 meter) nematode, liv- 

 ing in the water, inclosed in land and water insects and their larvae, as 

 well as in minnows and loaches; and the CucuUanus when young in 

 small crustaceans, and when fully grown in the perch and eel. In the 

 swimming-bladder of the salmonoids we find the Ancyranthus ; in the 

 stomach of the eel, the Filaria dentleulata ; in fish of the Oyprinus kind, 

 the Trichosoma tomentosum ; and in the plaice, the HeteraJiis foveola. 

 The large genus Ascaris has also many representatives in fish, for example, 

 Asearis truncatula, in the perch ; A. gasterostei, in the stickle-back ; A. 

 clavata, in the codfish, trout, and salmon ; A. mticronata, in the burbot 

 and pike; A. collaris, in the plaice; A. siluri, in the "Wels"; A. acus, 

 in the carp, pike, and trout; A. deritata, in the barbel; A. hirsuta, in 

 the smelt ; A. obtusocauda, in fish of the Coregonus kind ; A. cristata, in 



* As far as can be judged from the very incomplete description, the nnnsual mor- 

 tality of the crawfish noticed in several places {"Deutsche Ftscherei Zeitunf)," 1S79, p. 

 62) might be traced to the Echinorhyncus, perhaps E. polymorplius Br., E. Milariiis, 

 Zenker, E. Astaci fluvialis v. Siehold. When young they live in small crustaceans, and 

 when more developed in aquatic birds. These latter woiild, therefore, transmit the 

 infection. 



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