940 PROTOZOA AND OTHER ANIMALS 



and Hy pocomatidtum sphaerii Jarocki and Raabe, from the gills of fresh- 

 water mussels, reduction is far advanced to a thigmotactic zone restricted 

 almost entirely to the anterior half of the ventral side. Cilia are not lost 

 altogether in knov^n Hypocomidae. 



Syringo pharynx pterotracheae, which lives on the gills of the heteropod 

 Pterotrachea coronata either swimming free or fixed to epithelial cells 

 by the rostrum (Collin, 1914), was included by Kahl (1934) among 

 the parasitic gymnostomes. Probably, however, this is a hypocomid 

 ciliate, one with a general body ciliature like Ancistrocoma and Hypo- 

 cotnagahiia. Parachaenia myae, described from Mya arenaria by Kofoid 

 and Bush (1936), may also be a hypocomid, although attachment to the 

 cells of the gills of its host was not described. Chatton and Lwoff's 

 statement (1926, p. 351) about the prolongation of the anterior indi- 

 vidual in a spur covering the dorsal anterior region of the posterior 

 individual in binary fission of Ancistrocoma pelseneeri is in exact agree- 

 ment with the division process of Parachaenia myae. Another point of 

 agreement with Ancistrocoma is the type of conjugation. In the shape of 

 the ciliates, the unique attachment of the conjugants by the posterior 

 ends, and the shape and arrangement of the nuclei of P. myae, there is 

 almost complete agreement with A. pelseneeri as figured by Raabe 

 (1934a). 



Among the many genera of Hypocomidae, not all of which can even 

 be named here, the number of which we may expect will be markedly 

 reduced with further study, only the species of Hypocoma (Fig. 199G-I) 

 do not occur on bivalves or snails. They are parasites of Protozoa, and 

 are discussed elsewhere in this book (p. 1083). 



Hypocomidae are host-specific in marked degree. They are obligatory 

 parasites on certain individual or closely related molluscs, and do not 

 readily infect other molluscs. Jarocki (1935) found that Heterocineta 

 janickii, placed free-swimming into an aquarium with various molluscs, 

 would attach only for periods of from fifteen to eighteen hours in the 

 absence of its natural host Physa fontinalis. When various molluscs, in- 

 cluding Bithynia tentaculata, were put together in an aquarium, Hetero- 

 cineta krzysiki, though abundant on the body of Bithynia, did not infect 

 any other species. Sometimes two hypocomids are present on the same 

 host, and Raabe (1934a) noted that there seems to be a tendency to 

 inhibition by one parasite of the development of the other. 



