THE ONTOGENY OF THE PLEUREMBOLIC PROBOSCIS IN NUCELLA-LAPILLUS (GASTROPODA, MURICIDAE)

The ontogeny of the proboscis in Nucella lapillus was investigated usi ng light and scanning electron microscopy. The proboscis develops by e longation of the body wall surrounding the mouth, whilst the rhynchoco el is formed by invagination of the body wall surrounding the probosci s. Elongation of the snout during development of the proboscis results in the anterior movement of the anterior oesophagus and part of the m id-oesophagus (the valve of Leiblein) which is drawn through the circu m-oesophageal nerve ring. The acinous salivary glands and the radular sac also come to lie anterior to the nerve ring. The mid-oesophageal g land of Leiblein and the glandular dorsal folds are not drawn through the nerve ring, and develop behind it. The anterior oesophagus elongat es at a later stage of development to produce the oesophageal length r equired for extension of the adult proboscis. Modifications to this se quence of events, or changes in the rate of growth of the various part s of the foregut, might account for the differences between the neogas tropod and neotaenioglossan pleurembolic proboscis. The intraembolic p roboscis found in the Conoidea and the Pseudolivoidea may have been de rived via a modification of the developmental sequence which produces the muricoidean pleurembolic proboscis.