THE CILIATES OF STRONGYLOCENTROTUS DROBACHIENSIS : INCIDENCE, DISTRIBUTION IN THE HOST, AND DIVISION C. DALE BEERS Department of Zoology, University of North Carolina, Chapel Hill, and the Mt. Desert Island Biological Laboratory, Salsbiiry Cove, Maine Most species of sea urchins harbor in their alimentary tracts a characteristic fauna of ciliated protozoa. As Kirby (1941) points out, these ciliates appear to be obligatory inquilines ; at least there is no satisfactory evidence that they are nor-mally free-living forms which become established accidentally in sea urchins. Al-though their occurrence is well known, their general biology has been only meagerly explored. Little is known' regarding their host-specificity, mode of transmission, and affinities ; their taxonomy is confused, and their structure and division are inadequately described. In fact, it appears that representatives of only three genera have been subjected to critical morphological examination. These are Entor-hipidium and Lcchriopyla, which were studied by Lynch (1929, 1930) with special reference to ciliation, cytoplasmic inclusions and the neuromotor system, and Ento-discns, which was similarly studied by Powers (1933b). The conjugation of only one of them, Cryptochilidium echini, has been described (Dam, 1930), and none have been cultured. Thus the ciliates stand in need of much additional in-vestigation. The present study is a contribution to the general biology of the ciliates of Strongylocentrotus drobachiensis, the north-ranging "green urchin" common to both coasts of this continent and to Europe. The study makes no claim to com-prehensiveness, but is restricted to the aspects enumerated in the title. The studies of Powers (1933a) have shown that in the Bay of Funcly region at least seven species of ciliates occur in the digestive tract of this urchin. Some of them are of invariable occurrence ; others are found erratically. The seven ciliates and their present taxonomic status are as follows: (1) Entodiscns borcalis (Hent-schel, 1924). This large, common ciliate was first described by Hentschel under the name Cryptochilum boreale from specimens taken from Echinus esculentus in the Shetland Islands. Madsen (1931) also found it in S. drobachiensis at Fred-erikshavn, Denmark. He removed it from Cryptochilum and created the genus Entodiscns to receive it. (2) Madscnia indomita (Madsen, 1931). This slender, abundant ciliate was first described by Madsen as Entodiscns indomitus. Kahl (1934) removed it from this genus and established the genus Madsenia to receive it. (3) Biggaria gracilis (Powers, 1933). This ciliate was described by Powers under the name Cryptochilidium gracile. Kahl transferred it to the genus Biggaria, which he founded in 1934. This ciliate and the aforementioned two are tricho-stomatous holotrichs belonging to the Entorhipidiidae, according to Kahl (1934). (4) Plagiopyla ininuta Powers, 1933. This uncommon trichostomatous holotrich is referable to the Plagiopylidae. (5) Cyclidiwn stercoris Powers, 1935. This small hymenostomatous holotrich, a member of the Pleuronematidae, was first de-99
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