Ward. — Fish Parasites and Parasitic Diseases 223 



their universal presence in fish it appears probable that they 

 are rarely if ever of any marked disadvantage to their hosts. 

 The small encysted forms which appear as knots or coils 

 from one to several millimeters in diameter in the viscera, 

 the lining of the body cavity, or more rarely in the muscles 

 of the fish, are immature stages that are awaiting transfer 

 to some other host to complete their development. The 

 cysts are most abundant in small pan fish though not en- 

 tirely wanting in the largest predatory fish. 



These encysted worms have been reported often from 

 market fish, as by Leidy (1878) in the shad. While they 

 detract from the appearance of the flesh and interfere with 

 ready sale, they are not harmful to the fish and do not injure 

 its food value. 



The larger predatory fish contain more frequently the 

 adult stages of these worms as parasites free in the body 

 cavity or the intestine. Only rarely are the parasites abun- 

 dant enough to exercise any detrimental influence on the 

 activity or health of the fish. When smaller fish become 

 heavily infected, however, the draft on their energy may be 

 sufficient to produce serious consequences. I have observed 

 a heavy mortality among small fish, especially the stickle- 

 back, which was clearly due to parasitism by a nematode, 

 from one to several specimens of which were found coiled 

 up in the body cavity. The total mass of the parasites 

 equalled or approached that of the fish. In large fish the 

 number of such parasites which may be present without ex- 

 ercising any apparent effect on the welfare of the host is 

 often astonishing. 



The Acanthocephala are roundworms that possess a 

 proboscis covered with hooks which they drive into the wall 

 of the intestine and thus maintain their hold. They are 

 present at times in very large numbers so that they almost 

 occlude the lumen of the canal and the wall is badly dis- 

 torted by the numerous proboscides driven into it. At times 

 they bore their way through the wall into the body cavity. 

 Perhaps by virtue of their ability to make wounds in the 



