D1ST03EUM CIlilUCxEKUM. 275 



lying free among the muscles of the crayfish. This animal 

 possesses no trace of generative oi'gans. In 1880-81 C. O. 

 Harz ^ ascribed the disastrous epidemic " among the crayfish 

 of the Continent as being due to the presence of D. cirri- 

 gerum and D. isostomuni. In the same year the vieAv of 

 Harz was proved to be unfounded by G. Zaddach/ who 

 published without figures short descriptions of the anatomy 

 of the two trematodes, which are frequently found together 

 in the same crayfish. Zaddach regarded D. isostomnm as 

 being a sexless phase in the life-history of D. cirri gerum. 

 He propounded the following hypothesis. The cercaria 

 swimming in the water penetrate the skin of the crayfish, 

 become encapsuled in the muscles, and grow into the sexually 

 mature D. cirrigerum. The encapsuled animals after self- 

 fertilisation produce a heap of brown eggs and die. The 

 eggs become scattered among the muscles of the host, and 

 develop into the sexless unattached trematode D. isostomnm. 

 The crayfish containing the parasites is now supposed to be 

 eaten by some vertebrate (pike, eel, otter, etc.), in whose 

 intestine the trematode becomes sexually mature. Eggs are 

 laid and pass out of the vertebrate host. Zaddach imagines 

 that the earlier stages of the embryo are passed in a snail or 

 some other invertebrate in the typical manner. 



The evidence in favour of such a view is not very con- 

 vincing, and it would render the life-history of the species 

 highly remarkable in that there would be two sexual phases, 

 one in the crayfish and one in some vertebrate. 



Without further observations on the occurrence of D. 



> Harz, C. 0., " Eiue Distomatosis des Flusskrebses," ' Deutsche Zeitschr. 

 fiir Tluermed. u. vergl. Pathologie,' Bd. vii, 1881; also ' Oesterr-ungar 

 Fischerei-Zeitung' fiir 1880-81. 



- This epidemic is actually caused by the myxosporidian Thelohania con- 

 tejeani, Henneguy. 



^ Zaddach, G., " Uber die iia Flusskrebse vorkonimenden Distonium 

 cirrigerum, v. B., und D. isostomum, Rud.," ' Zool. Aiizeiger,' 1881. E. 

 Gaffron describes carefully the nervous system of D. isostomum, 'Zoolog. 

 Beitrage,' lirsg. von Anton Schneider, Bd. i, Breslau, 1884. 



