THE CILIATES OF STRONGYLOCENTROTUS DRÖBACHIENSIS: INCIDENCE, DISTRIBUTION IN THE HOST, AND DIVISION

¹ Department of Zoology, University of North Carolina, Chapel Hill, and the Mt. Desert Island Biological Laboratory, Salsbury Cove, Maine

Of 182 specimens of S. dröbachiensis, 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 in the alimentary tract and that they are not lost in great numbers at defecation. Counts of discharged fecal pellets and escaping ciliates indicate that an average of only one ciliate is lost per two fecal pellets. This rate of loss is low, in view of the great number of ciliates present per urchin. Thus infrequent divisions suffice to compensate for these moderate losses. B. gracilis is lost in greater numbers than any other species and must divide constantly in order to maintain itself.

At each division of B. gracilis and C. stercoris a mass of macronuclear material aggregates at the center of the elongated macronuclear membrane. This mass is discarded into the cytoplasm when the daughter macronuclei separate. A similar macronuclear reorganization usually accompanies the division of M. indomita.