938 PROTOZOA AND OTHER ANIMALS 



on the embryo (Cepede, 1910). The parasites do not act directly on the 

 embryos, but on the medium, which exerts an injurious effect on embryos 

 in the early stages. Abnormal embryos result, in which the shell is more 

 or less unrolled; not only may the shell be misshapen, but the cells that 

 secrete the shell may fail to function normally. 



The Hypocomidae (Fig. 199E-I) are true parasites, and occur mostly 

 in marine and fresh-water bivalves and snails. There is no mouth, but the 

 anterior end is provided with a short retractile tentacle. Normally the 

 ciliates are attached to the gills or skin of the mollusc, the tentacle being 

 embedded in an epithelial cell. The parasites obtain nutriment by ex- 

 tracting the contents of the cells to which they are attached, the tentacle 

 combining suctorial functions with those of attachment. The tentacle 

 continues in a tubular structure, extending more or less deeply into the 

 cytoplasm. In many Hypocomidae a fine inner canal has been observed 

 extending from the apex of the tentacle into the deeper cytoplasm. Ac- 

 cording to Chatton and Lwoff (1922c), this adherent organelle is de- 

 rived from structures of Ancistrumidae, where it is indicated in Ancis- 

 truma mytili and is well developed in A. cycUdioides. 



In relation to the attached parasitic condition of hypocomid ciliates 

 are the regression of the mouth and peristomal ciliature and the reduc- 

 tion of the general ciliature. The former structures have for the most 

 part already disappeared. There is no mouth, but Chatton and Lwoff 

 (1924) stated that in some genera there are residual segments of the 

 adoral ciliary zone. 



In reduction of the general ciliature there is in Hypocomidae a well- 

 integrated series. The body of Hypocomagalnia dreissenae Jarocki and 

 Raabe, 1932, is covered with cilia except for a small ventroterminal area. 

 Anctstrocoma pelseneer'i Chatton and Lwoff, as figured by Raabe 

 (1934a), has a larger cilia-free area, occupying a large part of one side 

 of the body, and an anterolateral peristomal fringe. Perhaps the less 

 firm fixation of these forms is also a phylogenetically primitive character. 

 Raabe stated that Anchtrocoma adheres rather weakly to the gills of its 

 host and separates readily. Hypoconiagalnia swims more rapidly than 

 some other ciliates of the group. Reduction of the ciliature continues 

 through Hypocomides Chatton and Lwoff and Hypocoma Gruber, in 

 which it occupies the inner area of the ventral surface of the body. In 

 Heterocineta {==Hypocomatomorpha) un'wmdavum Jarocki and Raabe 



