A Study of the Parasites of the Unionidte. 413. 



III.), — forty-one per cent, were parasitized by Aspidogaster, 

 eighteen per cent, by Cotylaspis, and thirty-seven per cent, 

 by Atax — as shown in Table IV. It may be seen from the 

 tables under discussion, as well as from Table II., that there 

 is a marked difference between the several host species in 

 capacity for infestation in both kind and degree. 



The range in number of parasites infesting one host, and 

 their average number, may depend to some extent upon the 

 size of the host, — Anodonta suborbiculata and A . corpulenta 

 showing high numbers and Strophitus edentula and Anodonta 

 imbecilis low, — and this applies with force in case of occupa- 

 tion of the pericardial and nephridial cavities by Aspidogaster, 

 where the volumes of the organs closely limit the possible 

 number of invading parasites. But size is not the sole 

 determining factor, else Lampsilis lutcolus and L. anodontoi- 

 des, L. ligamentinus and L. alatus, Quadrula multiplicdta and 

 Lampsilis ventricosus, and Quadrula plicata and La mpsilis gra- 

 cilis should harbor similar, rather than so widely different, 

 numbers of parasites, and little Lampsilis parvus should not 

 show such large infestation and such a wide range in the 

 number of parasites harbored. 



The tables seem to indicate in the different species a 

 general correspondence between the frequency of infestation, 

 the variety of parasites, and the average number of individual 

 parasites harbored by a given host. Thus Quadrula tuber- 

 culata, Anodonta suborbiculata, A. corpulenta, Lampsilis 

 ellipsis, L. ventricosus, L. alatus, and L. gracilis, all figuring 

 largely in the tables, are frequently parasitized, carry a large 

 variety of parasites, and, in proportion to their size, a high 

 average number individually ; while the statistics concerning 

 Quadrula nmltlplicata , Q. trigona, Q. ebena, Unlo gibbosus, 

 Obliquaria refiexa, and Plagiola elegans show a like uniformity 

 in infrequent infestation, little variety in kinds of parasites, 

 and a low average number harbored by the individual host. 



In the light of the latest views upon the natural classifica- 

 tion of the Union id, c it may be said that closely related 

 species exhibit somewhat similar capacities for infestation. 

 In general the species of Anodonta and also those of Lamp- 



