OBSERVATIONS ON THE TECTIBRANCH RINGICULA BUCCINEA (BROCCHI)

Ringicula buccinea is an opisthobranch mollusc with a comparatively large external shell which burrows in muddy sand using its foot and cephalic disc. The latter helps to exclude sand from the mantle cavity, is produced into an inhalant siphon dorsally and is fused with the foot laterally. The gill is reduced and the main pallial water current is an exhalant one. There is no exogyrous caecum. Glands open just within the lips and the buccal cavity contains paired jaws and a reduced buccal mass and radula. Into it open the small salivary glands. The oesophagus has neither glands nor crushing plates. The stomach is large with the oesophagus, intestine and liver ducts opening close together to a thin‐walled anterior chamber, whereas a larger posterior chamber has thick, muscular walls and a cuticularized epithelium. Interstitial copepods and Foraminifera are collected for food and crushed in the triturating region. The rectum and pallial genital oviduct lie in the floor of the mantle cavity. A single duct carries both sperm and ova to the mouth of the cavity a bursa copulatrix and receptaculum are associated with it. The penis is retracted into a sheath which opens on the head and is associated with a prostate gland. Discussion of these aspects of the animal's structure confirms its position as a primitive tectibranch which is specialized for burrowing and also shows affinities with the basommatophoran pulmonates.