The ontogeny of the pleurembolic proboscis in Nucella lapillus (Gastropoda: Muricidae)

The ontogeny of the proboscis in Nucella lapillus was investigated using light and scanning electron microscopy. The proboscis develops by elongation of the body wall surrounding the mouth, whilst the rhynchocoel is formed by invagination of the body wall surrounding the proboscis. Elongation of the snout during development of the proboscis results in the anterior movement of the anterior oesophagus and part of the mid-oesophagus (the valve of Leiblein) which is drawn through the circum-oesophageal nerve ring. The acinous salivary glands and the radular sac also come to lie anterior to the nerve ring. The mid-oesophageal gland of Leiblein and the glandular dorsal folds are not drawn through the nerve ring, and develop behind it. The anterior oesophagus elongates at a later stage of development to produce the oesophageal length required for extension of the adult proboscis. Modifications to this sequence of events, or changes in the rate of growth of the various parts of the foregut, might account for the differences between the neogastropod and neotaenioglossan pleurembolic proboscis. The intraembolic proboscis found in the Conoidea and the Pseudolivoidea may have been derived via a modification of the developmental sequence which produces the muricoidean pleurembolic proboscis.