AILMENTS AND DISEASES 



Probably the most frequent forms of Cestode parasites are Schistoceph- 

 alus solidus and allied species, which occur in many freshwater fishes in 



FIG. 87 Stickleback affected with Schistociphalus solidus; showing enlargement of side and abdomen. 



Slightly enlarged. 



immature forms. Figs. 86, 87 and 88. As adults they have fish-feeding 

 birds and mammals as hosts. 



FIG. 89 Ascaris acus, a Nematod 

 parasite, i . Immature forms, enlarged. 

 2. Subsequent adult form, enlarged. 



FIG. 88. Section of a Stickleback, 

 showing cysts of Schistociphalus solidus. 

 Slightly enlarged. 



Nematoda or Roundworms. This group comprises the round 

 and thread worms. They include a large number of families which occur 

 abundantly in all genera of the Vertebrates and are more numerously and 

 uniformly distributed than the individual members of the other parasitic 

 orders. They have not apparently had the attention of Helminthologists 

 that has been given to many other groups and the literature is fragmentary 

 and widely scattered. Mention will here be made of the six principal 

 families and the more generally distributed species. 



AscARiD^ AND Cheiricanthid^. These parasites inhabit higher 

 Vertebrates; principally birds and mammals, but two Nematods, bearing 

 close resemblance to Ascaris teniussima, have been found in Western 

 trout. Ascaris acus in the adult form is encysted in the intestines of pike, 

 bass and trout. Fig. 89. Its immature existence is spent in the minnow, 

 dace and other Cyprinidae and with them is introduced into the subsequent 

 host. 



Cucullanid^. Several species of these parasites occur in freshwater 



149 



