404 Illinois State Laboratory of Natural History. 



infestation by Bucephalus, has already been explained as the 

 result of the unsexing of the host. 



Though this study was instituted primarily with the 

 trematodes alone in mind, record was made of all parasites 

 whose presence did not appear to be accidental. I have 

 presented in Table II. a concise record of these parasites and 

 of the species and number of their hosts, while the relations 

 of the one to the other are set forth in the discussion which 

 follows. 



Aspidogaster conchicola von Baer, the most common parasite 

 of the Uiiionidcv, is confined for the most part to the peri- 

 cardial and nephridial cavities of the host. In four hundred 

 and thirty-five cases- it was found in the pericardium only, 

 in seventy in the kidneys only, and in one hundred and 

 thirty-four cases both cavities contained the parasite. In 

 only one host species showing any considerable degree of 

 infestation, Lampsilis parvus, — where twenty out of thirty- 

 one individuals examined were parasitized, — were the flukes 

 restricted wholly to one cavity (the pericardium), and here 

 the small size of the host may perhaps account for such 

 restriction. As a rule, though there are many exceptions, 

 flukes appear in both chambers only when the parasites are 

 very numerous ; and as the number in the pericardium is 

 usually much larger than that in the kidneys, and as the 

 pericardial infection is the more frequent, it would seem that 

 only in excessive parasitism is the nephridial cavity invaded. 

 A single Aspidogaster was found encysted in the lateral wall 

 of the visceral mass of the host. In four cases only, in all 

 of which the pericardium was ruptured in opening the shell, 

 were individuals of this species detected in other than the 

 usual localities, and then their positions were always such as 

 to suggest escape from the broken pericardium. This para- 

 site was most frequently found in the adult stage, but eggs 

 and embryos in abundance and young of varying sizes were 

 found when the parasitism was considerable. The presence 

 of the mature trematode in the pericardium and of eggs 

 within the nephridia is not infrequent. 



Cotylaspis in sign is Leidy is found adherent to the surface 



