ON A NEW GENUS AND TWO NEW SPECIES OF PROSOBRANCHIA (MOLLUSCA), PARASITIC ON THE TROPICAL SEA URCHIN ECHINOMETRA MATHAEI

ABSTRACT Males and females of the parasitic prosobranch Monogamus entopodia n. gen., n. sp. occur together within the same tube foot of Echinometra mathaei in the Gulf of Elat (= Gulf of 'Aqaba), Red Sea. The anatomy of the parasite and its relation to the host have been studied, and observations on its biology are given. The snout has a most unusual shape and is deeply inserted into the tube foot, but does not penetrate the hydro-vascular system, the tube foot component of which degenerates as a result of parasitization. Males and females of another species, Monogamus interspinea n. sp., described from a few Pacific Ocean localities, live close together on the aboral side of the test of Echinometra mathaei. Monogamus is probably related to Robillardia Smith, which lives in the rectum of Echinometra.