PROTOZOA AND OTHER ANIMALS 935 



in that of all the Thigmotricha, insofar as a knowledge of geographical 

 distribution and host-specificity is concerned. 



In the mantle cavity of the hosts, some species are not localized, 

 whereas others are. Kidder (1934a) found C. curtus and C. magna on 

 all exposed surfaces and also swimming freely in the mantle fluids; 

 C. anodontae, on the other hand, he found to be invariably localized 

 on the nonciliated surface of the palps. The cilia of the flat left side 

 (left if, with Kahl and Raabe, we consider the flattening to be lateral; 

 according to De Morgan and Kidder, it is dorsoventral, and the attach- 

 ment is by cilia of the ventral surface) are thigmotactic. The thigmotactic 

 area usually covers the whole broad side, but in C. discophorus there is 

 more specialized adhesive apparatus, a circular, sharply outlined area, 

 which occupies only part of the left side, is markedly concave, and is 

 provided with difl^erentiated cilia (Raabe, 1934b). C. discophorus swims 

 slowly, and often fastens itself firmly by the thigmotactic region. C. 

 mytili (Fig. 199A) also swims about or clings firmly to surfaces (Kid- 

 der, 1933a). C. anodontae on ElUptio cotnplanatus seems to be most 

 markedly thigmotactic (Kidder, 1934a), remaining quiet, attached to 

 the surface of the palp. 



Kidder (1933a) found the food vacuoles of C. mytili {Morgania 

 mytili, according to Kahl, 1934) to contain plankton organisms, includ- 

 ing algae, and sperm cells of the host. C caryoclada [Morgania caryo- 

 clada, according to Kahl) contained mostly algae (Kidder, 1933d). 

 Other species contained algae, bacteria, and sloughed-off epithelial cells. 

 The relationship appears to be simple commensalism, but Kidder 

 ( 1934a) , finding only well-preserved epithelial cells in the food vacuoles 

 of C. magna, was "a little in doubt as to its purely commensal role." 



Kidder (1934a) remarked that there is a fair degree of host- 

 specificity. In nature certain species are characteristic of certain molluscs; 

 and the faunules may difl^er, even though in nature the hosts are very 

 closely associated. Rarely there are as many as three species in one host. 

 A number of species have been found in only one or a few related 

 hosts, but this may be a consequence of the relatively few examinations. 

 A cosmopolitan distribution is characteristic of such species as C. curtus, 

 reported from various fresh-water clams in Europe, the eastern United 

 States, and Japan. Morgania mytili is a commensal of Mytilus edulis in 

 various localities on both sides of the North Atlantic. 



