A NEW COMMENSAL HYDROID OF THE MANTLE CAVITY OF AN OYSTER x N. T. MATTOX AND SEARS CROWELL Department of Biology, College of Agriculture and Mechanic Arts, Mayaguez, Puerto Rico, and Department of Zoology, Indiana University, Blooinington, Indiana During the course of studies on the biology of the "mangrove oyster," Crassostrea rlihophorae (Guilding), in Puerto Rico, a number of oysters were found with the mantle cavity infested by an undescribed commensal hydroid (Mattox, 1949). The unusual habitat of this hydroid and its correlated adaptive features are not entirely unique, for two similar hydroids have been found in Italy, both also living in the mantle cavity of edible pelecypods. The hydroid here considered is recognized as belonging to the same genus as those found in Italy and is designated as E-ngymtian-tJiea ostrcanun. THE GENUS EUGYMNANTHEA Palombi (1936) established the genus Eugymnanthea in naming a new species of hydroid found within the mantle cavity of the lamellibranch, Tapes deciissatns, in Lago Fusaro, near Naples. This hydroid, Eugymnanthea inquilina Palombi is the generotype by monotypy. Cerruti (1941) described a similar hydroid from Mytilus galloprovincialis at Taranto. Unaware of Palombi's work he named this Mytilhydra poliuiaiitii, but upon learning of E. inquilina altered the name of his species to Eugymnanthea polimantii. Our species is similar to the other two and must be placed in the same genus. The following are the features common to the three species of Eugymnanthea and constitute a definition of the genus. Athecate, solitary polyps, lacking hydro-rhizae and gonothecae, possessing a basal disc, a single whorl of tentacles, and pro-ducing from near the base, either sporosacs, or medusae of the leptomedusan family Eucopiidae. The three known species are commensal within the mantle cavity of pelecypods. Eugymnanthea ostrearnm nov. sp. Description of the hydroid (Fig. 1 A). The polyps are simple, solitary, taper-ing toward the base, having a maximum length of 3.8 mm. (the average of 20 speci-mens, taken at random, was 1.54 mm.), having a maximum diameter of the column of 0.35 mm. There are as many as 35 filiform tentacles (average 30) in a single whorl. There is a slight intertentacular membrane which, on the largest polyp, was 0.18 mm. in width. The hypostome is a slightly elevated cone which does not extend beyond the limits of the intertentacular membrane. A striking feature is the basal or pedal disc, a flattened, slightly concave expansion of the base of the column. 1 Contribution from the Department of Biology, College of Agriculture and Mechanic Arts, Mayagucz, Puerto Rico (Mattox), and Contribution No. 474 from the Department of Zoology, Indiana University, Bloomington, Indiana (Crowell). 162
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