A NEW COMMENSAL HYDROID OF THE MANTLE CAVITY OF AN OYSTER

¹ Department of Biology, College of Agriculture and Mechanic Arts, Mayaguez, Puerto Rico, and Department of Zoology, Indiana University, Bloomington, Indiana

1. Eugymnanthea ostrearum nov. sp. is a small, solitary hydroid living, attached by a basal disc, within the mantle cavity of the oyster Crassostrea rhizophorae (Guilding), in Puerto Rico. Two similar species have been described from Italy.

2. Its taxonomic position as a hydroid is somewhat uncertain since it lacks nearly all of the characters used in hydroid classification. It produces a leptomedusa similar to Eucope, and as a medusa belongs to the family Eucopiidae, subfamily Obelinae.

3. It shows the following features which may be regarded as adaptations appropriate to its commensal existence within the mantle cavity of the oyster: basal disc, absence of perisarc and stolons. Since the polyps are solitary there can be no gonangia.

4. Polyps are produced by budding followed by a fission of the basal disc. The development of the eggs has not been studied.

5. The development of the medusa is not unusual except in the way in which the mantle is established. It arises in a more sharply defined manner than that described in other forms by previous observers.