The Eggs and Early Larvæ of Two Commensal Gastropods, Stilifer stylifer and Odostomia eulimoides

Extract

Stilifer stylifer (Turton) has been recorded from Plymouth many years ago living on Psammechinus miliaris (=Echinus) and Echinus esculentus; also from several other British localities (see Jeffreys, 1867, IV, p. 196). Jeffreys describes the spawn and larvæ hatched from it, but apparently these have never been figured. It had not been rediscovered at Plymouth until 1931 when in August Mr. A. D. Hobson found several specimens of Psammechinus miliaris from deep water beyond the Eddystone with these small commensal molluscs on them. In the first lot of about a dozen Psammechinus, three were covered; with the spawn of Stilifer, one having one mollusc and two having two. In a second lot taken a few days afterwards, five were covered with spawn, one having one mollusc, two having two, and two having three. These were placed in a plunger-jar, but unfortunately most of the Stilifer left the Psammechinus and were presumably eaten by them as pieces of their shells were found. They probably did not like their surroundings. Some of the spawn was removed from the Psammechinus and placed in a fresh plunger-jar with some Nitzschia where some of the larvæ hatched out. A Psammechinus was introduced in the hope that the larvæ might settle down, but they all died after living only a few days.