A Stiidii of the Pio-asites of the Unionlda'. 418 



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

 eighteen per cent, by Coti/Iaspis, 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, — Atwdontd suhorhlcidata and A. corpuleiita 

 showing high numbers and StropJiitiis edentida and Auodoiita 

 'nuhec'diii low, — and this applies with force in case of occupa- 

 tion of the pericardial and nephridial cavities by Aspidof/aster, 

 where the volumes of the organs closely limit the possilde 

 number of invading parasites. But size is not the sole 

 determining factor, else Lump^^ill^ luteolus and L. (inodontol- 

 des, L. li(/(ime)tthiHS and L. (datiis, Qnddndd midtijdicata and 

 Lampsilia veiitrieosiis, and Quadrida plieata and Ldiiipsills (jra- 

 cUis should harbor similar, rather than so widely different, 

 numbers of parasites, and little LanipsUis p>'(Trus 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 tuhcr- 

 ridata, Anodonta suborhietdata, A. covpulotta, LampsiVis 

 ellipsis, L. ventricosus, L. <d<ttus, and L. (irdedis, all figuring 

 largely in the tal)les, are frequently parasitized, carry a large 

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

 average number individually ; while the statistics concerning 

 Qmidrida imdtipUcatu, (J. trigona, Q. ehena, Unio gihhosns, 

 Ohliquaria rejiexa, and Vlagiola 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 Vnionhhc 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- 



