THE ANATOMY OF THE PROSOBRANCH CIRCULUS STRIATUS (PHILIPPI) AND A REVIEW OF ITS SYSTEMATIC POSITION

SUMMARY The animal described above agrees in its conchological characters with Tornus duminyi Requien as described by Jeffreys, and listed by Winckworth as Circulus striatus (Philippi). Neither shell nor radula agrees with the Circulus striatus (Philippi) of Thiele. The mantle cavity contains a linear osphradium and a large monopectinate gill; the single kidney opening is posterior; the anus half‐way along the right side within the hypobranchial gland. In the structure of the jaws, radula and alimentary canal there is close resemblance to the rissoids. The single kidney on the left of the rectum separates the renal oviduct from the pericardium. There is no gonopericardial canal: the renal oviduct opens into the kidney. In the male the penis arises between the two cephalic tentacles and the vas deferens, a closed duct, opens at its tip; there is a prostate with an opening to the mantle cavity. The glands of the oviduct are confined to the posterior end of the mantle cavity; the duct opens some distance behind the anus. The plan of the female reproductive system conforms with that of other mesogastropods. The nervous system is characterized by a concentration of the ganglia with the exception of the viscerals. The affinities of Circulus striatus with Tornus subcarinatus, the rissoids and the genus Cyclostrema are considered. Its anatomy supports the conchological evidence for its classification close to the family Rissoidae. It should not be included in Thiele's family Adeorbidae. Correlation between the small size of this species and certain details of its anatomy is discussed. I wish to express my thanks to the Research Board, University of Reading, for the grant to cover travelling expenses to St. Jean‐de‐Luz.