A Study of the Parasites of the Unionidce. 405 



of the host in the angle between the inner gill and the 

 visceral mass. Its range is usually restricted to this axilla, 

 and the number infesting one host is small. In one case, 

 however, — that of Anodonta corpulenta, recorded by Dr. 

 Kofoid. — where the number reached the unparalleled extreme 

 of ninety-two, the flukes extended well out upon the inner sur- 

 face of the gill ; and in another, under my own observation, 

 some of them were crowded down upon the abdominal surface. 

 In A. suborbiculata, in which Cotylaspis attains its maximum 

 average, thirty-eight to each host, not only are the axillge and 

 the adjacent surfaces of both inner gill and visceral mass 

 invaded, but some are usually found within the tubes of the 

 inner gill, and occasionally even in those of the outer gill. 

 Such migration from the usual seat of infestation to immedi- 

 ately adjacent regions is perhaps to be expected in cases of 

 overcrowding such as are instanced. In a single Lampsilis 

 ellipsis, one Cotylaspis unmistakably occurred in the peri- 

 cardium along with twenty-three specimens of Aspidogaster. 

 Since Cotylaspis normally frequents the region of the nephrid- 

 ial openings, an invasion of the pericardium by way of the 

 nephridia might not be impossible. All the Cotylaspis found 

 were adults varying little in size. Eggs were not infrequently 

 observed in the surface slime collected in the vicinity of the 

 parasites. 



Four forms of Distomidce have been found, probably of as 

 many different species, all immature, and none sufficiently 

 developed for specific determination. One of these forms — 

 referred to in the table as "Free Distomata" —is found in 

 loose salmon-colored masses either upon or slightly w ;t hin 

 the tissue of the mantle, along or. near the dorsal fold. In 

 Quadrula, Unio, Plagiola, and Lampsilis this parasite is most 

 frequently located immediately between the cardinal teeth, 

 less commonly between the lateral teeth, or, again, upon 

 the sides, extending over the external surface of the mantle 

 on a line parallel to its attachment to the viscera. In 

 the genera Strophitus and Anodonta the distribution of this 

 parasite is lateral, as just described, often extending over the 

 mantle surface like a large widely-open inverted V, with its 



