74 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



is abundant throughout the southern Appalachians. Our information 

 suggests that C. fallax has reached the James and Roanoke from the 

 New, but it does not indicate any choice between the New or the 

 Tennessee as the ancestral home of the species. In summary, the 

 presence of the branchiobdellids in the Mountain Lake region may 

 be explained as follows : 



Ankyrodrilus koronaeus — originated in or near the area, but derived from a 



Cumberland Plateau group by way of the Tennessee drainage. 

 Ankyrodrilus legaeus — by migrations from the Cumberland Plateau region by 



way of the Tennessee drainage. 

 Bdellodrilus illuminatus — unknown origin (ancient, widespread). 

 Cambarincola branchiophila — New River endemic, relict survivor of a once wider 



range. 

 Cambarincola fallax — a southern Appalachian species (now widespread in eastern 



highlands) that reached James and Roanoke from New. 

 Cambarincola heterognatha — New River native. 



Cambarincola holostoma — James River native, evolved from C. philadelphica stock. 

 Cambarincola ingens — by migrations from the south, headwaters of New or 



Tennessee. 

 Cambarincola philadelphica — Appalachian (widespread) . 

 Pterodrilus alcicornus — Tennessee drainage. 

 Xironodrilus formosus — relict of ancient Teays River fauna. 

 Xironogiton instabilius — unknown origin (ancient relict). 



It has been pointed out above that evidence exists that most of 

 the ostracods tend to favor one or more crayfish hosts. The extreme 

 of this favoritism is demonstrated in the relationship between C. 

 carolinus and its two "riders," As. asceta and Dt. chalaza, in which 

 the associations appear to be absolute for the ostracods. (As has 

 already been suggested, the "favoritism" probably has more to do 

 with the environment of the host than with physiological relation- 

 ships usually associated with "host specificity.") The opposite 

 extreme is demonstrated in an apparent lack of discrimination on 

 the part of the ostracod Dn. ileata that infests all the crayfishes in 

 the area except C. carolinus and is associated with it in other areas. 

 The affinities of other ostracods for one or two particular hosts seem 

 to lie somewhere between these two extremes. 



What the nature of the ties is between the ostracod symbionts 

 and the crayfish hosts and between the branchiobdellid symbionts 

 and the crayfish hosts has not been critically analyzed. It cannot 

 be said that the ostracod relationship is parasitic even in the sense 

 that they have free transportation and pasturage, for they might 

 well be paying a service charge in ridding the crayfish of foreign organic 

 matter that adheres particularly to the setiferous areas of the exo- 

 skeleton. While they may not do a thorough cleaning job, the ac- 

 cumulation of such matter is probably retarded. 



