256 American Fisheries Society 



It was not until the present summer that this puzzling 

 rudiment was understood, when the adult stage was found 

 in the loon. 



The prevalence of this form of parasitism among the 

 dinners and tautog from certain localities is very great. 

 Neither is it unusual to find individual cases of extreme 

 abundance. In badly infested fish, not only the fins but the 

 entire surface of the body, including the cornea of the eye, 

 is thickly peppered with the cysts. Whenever a fish is thus 

 badly infected the prevailing color is blue. Some idea of 

 the great numbers of these cysts in extreme cases may be 

 gained from the count given for the corneas of a tautog, 

 one of the corneas being figured in the report cited above. 

 The count showed 74 cysts in the cornea of one eye and 81 

 in the cornea of the other. 



It is not at all improbable that a systematic study of these 

 cysts will reveal trematodes of different species. So far as 

 my examination has gone I am sure of only one species. 



While examining a loon at Woods Hole on July 24, 

 1911, I found a very large number of small trematodes in 

 the intestine which resembled the forms which I have ob- 

 tained from cysts in the skin of the cunner and other fish 

 of the Woods Hole region. Among them were young forms 

 without ova. When these are compared with larvae from 

 the dermal cysts of the cunner the identity is established be- 

 yond doubt. Both are characterized by the same kind of 

 oral sucker, pharynx, esophagus and intestine. The out- 

 line is similar in each and the surface of each is covered 

 densely with very minute spines of like appearance. In- 

 stead of a ventral sucker a genital sucker is present in the 

 adult. This median genital sucker made clear a heretofore 

 baffling rudiment in the encysted larvae which had been in- 

 terpreted, not with entire satisfaction, to be the ventral 

 sucker of a distome. A few examples of larvae from cysts 

 were obtained in which the rudiments of the testes and ovary 

 could be distinguished, and were seen to occupy the same 

 relative positions which they hold in the adult worms. 



