The ciliates of Strongylocentrotus dröbachiensis; incidence, distribution in the host, and division.

Of 182 specimens of S. drobachiensis, all were infected with Entodiscus borealis and Madsenia indomita; 181 with Biggaria gracilis; 98 with Cyclidium stercoris; 28 with Plagiopyla minuta; 24 with Euplotes sp.; and four with Trichodina sp.E. borealis occurs primarily in the stomach, though it extends into the intestine and rectum; B. gracilis, almost exclusively in the rectum; the five remaining species, chiefly in the intestine, less commonly in the rectum.Although E. borealis, M. indomita and C. stercoris were present in immense numbers, they were rarely found in division. The evidence indicates that division in these ciliates, and probably in P. minuta, is a cyclical phenomenon; short periods of intense divisional activity appear to alternate with long periods of nondivisional life. B. gracilis, to the contrary, was dividing in nearly every urchin.The ability of some of the ciliates to maintain themselves in enormous numbers in the absence of frequent divisions indicates that they do not perish readily ...