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

The ontogeny of the proboscis in Nucclla lapillus was investigatedusing light and scanning electron microscopy. The proboscis develops by elongation of the body wall surroundingthe mouth, whilst the rhynchocoel is formed by imagination ofthe body wall surrounding the proboscis. Elongation of the snoutduring development of the proboscis results in the anteriormovement of the anterior oesophagus and part of the mid-oesophagus(the valve of Leiblein) which is drawn through the circum-oesophagealnerve ring. The acinous salivary glands and the radular sacalso come to lie anterior to the nerve ring. The mid-oesophagealgland of Leiblein and the glandular dorsal folds are not drawnthrough the nerve ring, and develop behind it. The anterioroesophagus elongates at a later stage of development to producethe oesophageal length required for extension of the adult proboscis.Modifications to this sequence of events, or changes in therate of growth of the various parts of the foregut, might accountfor the differences between the neogastropod and neotaenioglossanpleurembolic proboscis. The intraembolic proboscis found inthe Conoidea and the Pseudolivoidea may have been derived viaa modification of the developmental sequence which producesthe muricoidean pleurembolic proboscis. (Received 10 May 1996; accepted 15 August 1996)