PROTOZOA AND OTHER ANIMALS 943 



like structures, producing a plaited surface suggesting Aspidogaster to 

 Raabe. At the anterior end is a beak-like process, which is embedded in 

 the cells of a filament, whereas the plaited surface adheres to another 

 filament. Nutrition is osmotic. 



PTYCHOSTOMIDAE 



The holotrichous ciliates of the family Ptychostomidae are considered 

 by some to be related to the Thigmotricha, if indeed they do not belong 

 in that group. Beers (1938b), following Jarocki (1934), gave prefer- 

 ence to Hysterocinetidae Diesing, 1866, as having priority over Ptycho- 

 stomidae Cheissin, 1932. There is, however, no general recognition of 

 priority in family names, but these are based on the name of the type 

 genus. In this instance it seems that Ptychostomum Stein, I860, the first 

 described, best known, and largest genus, should not be supplanted as 

 the type by Hysterocineta Diesing, 1866. Rossolimo (1925) suggested 

 that the Thigmotricha and Ptychostomum represent two parallel evolu- 

 tionary series, derived from the same group of free-living organisms, but 

 adapted in somewhat different ways to attachment and the requirements 

 of sedentary life. 



The family Ptychostomidae now includes some eighteen species, of 

 which nearly half are from Lake Baikal. The ciliates occur in the in- 

 testine of fresh-water oligochaetes, except for Hysterocineta eheniae de- 

 scribed by Beers (1938b) from a terrestrial oligochaete, and three species 

 from the intestine of gasteropods. In oligochaetes, Ptychostomidae are 

 associated with astomatous ciliates. Cheissin (1932) remarked that there 

 is a tendency for Astomata to be located more anteriorly in the intestine, 

 whereas Ptychostomidae occur in the posterior part. Beers found that 90 

 percent of H. eheniae are localized in the third quarter of the gut of 

 Eisenia lonnbergi; and an astomatous ciliate occurs more anteriorly. 

 Cheissin (1928) stated that "L<^^(9/?j/j" (=zHysterocineta) benedictiae is 

 found in the mantle cavity (?) of Benedictia baikalensis; later (1932) 

 he wrote that that ciliate occurs mostly in the intestine and enters the 

 mantle cavity seemingly only accidentally. 



It may be expected that study of fresh-water oligochaetes in various 

 parts of the world will greatly increase the size of the group. There may 

 then be a tendency to greater subdivision, but at present there are only 

 two genera, Ftychostomum and Hysterocineta, hada Vejdovsky being a 



