70 THE CRAYFISH [CH. II 



the encysted larvae of an Acanthocephalan, Echinorhynchus 

 polymorphus. This parasite becomes sexually mature in 

 the intestines of ducks, swans, geese and other birds of 

 similar habits. The adult male is about 5 mm. long, the 

 female about 25 mm. Two species of Trematodes, Disto- 

 mum cirrigerum and D. isostomum, are frequently found 

 among the viscera. The former of these is usually in an 

 encysted condition, the cysts varying from 0'2 mm. to 

 1*75 mm. in diameter. Zaddach 1 has observed self- 

 impregnation occur in this species: the eggs are laid in 

 the cyst; the parent dies and the eggs are scattered 

 within the body of Astacus by the decay of the cyst. It 

 is not known how the crayfish first becomes infected with 

 the parasite. 



Geographical Distribution. The genus Astacus is found 

 in Europe and Western Asia as far south as the Aral and 

 Caspian Seas, and also in America in the streams on the 

 west of the Rocky Mountains. 



1 Zool. Anz. iv. 1881, pp. 398, 426. 



