920 PROTOZOA AND OTHER ANIMALS 



Powers (1933a) occurred in only about 10 percent of Strongylocentrotus 

 drobachiensh, and then there were not more than twelve in a host; 

 whereas some ciliates have been found in all sea urchins of the species. 

 Most often the incidence is not 100 percent. 



Powers (1933a) pointed out the existence of two groups of ciliates in 

 sea urchins. One group contains diverse species with many free-living 

 congeners, which he regarded as chance or vagrant forms that were en- 

 gulfed with food and survived; the other consists of obligatorily endozoic 

 species. The members of the first group are "apparently free-living and 

 only occasionally or accidentally associated with their host." There is, 

 however, no evidence in the literature that many, if any, of the intestinal 

 ciliates in sea urchins are accidentally introduced free-living forms. 

 Though there are species belonging to genera of which most members 

 are free-living, that in itself is no indication that they are not obligatory 

 inquilines. 



Colpidlum echini Russo, found also by Powers (1933a) in all speci- 

 mens of Strongylocentrotus lividus examined at Naples, probably, ac- 

 cording to Kahl (1934), is not a Colpidium. Uronema socialis, described 

 by Powers (1933a) from S. drobachiensis, was later renamed by him 

 (1935) Cyclidium stercoris. Kahl (1934) doubted the generic assign- 

 ment of Colpoda fragilis, described by Powers (1933a) from Tox- 

 opneustes variegatus of Beaufort, North Carolina. These forms, which 

 Powers mentioned in the occasional associate group, together with 

 Plag/opy/a, may be obligatory commensals. The Euplotes sp. found by 

 Powers (1933a) in the gut and outside of S. drobachiensis is possibly 

 an accidental invader. He also reported Trichodina from the sea urchin 

 and in seaweed. 



Cyclidium stercoris, which occurs in great abundance in 5". drobachien- 

 sis, will live and reproduce in sea water (Powers, 1933a); but it is not 

 known that it does so under natural conditions. "Colpoda" fragilis, on 

 the other hand, is very sensitive to changes in its environment. Many of 

 the ciliates can survive for more or less prolonged periods outside of the 

 host. Entodiscus borealis, one of the strictly endozoic forms, was kept in 

 sea water from fifteen to twenty-three days (Powers, 1933b). 



Species of Cyclidium occur also in sea urchins of China (Nie, 1934) 

 and Japan (Yagiu, 1933, 1934). Several species of Anophrys have been 

 reported from various echinoids. There is only one free-living species 



