The fine structure of the excretory and venous systems of the neogastropods Nucella lapillus and Buccinum undatum and its functional and phylogenetic implications

Abstract

The fine structure and function of the excretory system and associated veins of the muricid Nucella lapillus have been studied and compared with those of the buccinid Buccinum undatum. It is argued that in both species the nephridial gland vein is a functional unit with the auricle as it is in Littorina, and that its sinuses provide a compensation sac for blood displaced from the mantle skirt on retraction of the snail into the mantle cavity. This may be typical of marine caenogastropods. The epithelium lining the tubules of the gland recaptures organic solutes from the urine at the start of the systemic circulation, and phagocytes in its sinuses (the blood gland) abstract macromolecules and toxins from the blood. The development of two different types of folds on the dorsal wall of the kidney in neogastropods and other carnivorous neotaenioglossans is correlated with adaptations of the foregut to diet and with feeding mechanisms in which a retractile proboscis is accommodated in the cephalic haemocoel. It does not necessarily reflect phylogenetic relationship. On the basis of the observations reported here it is concluded that the dorsal afferent renal vein and secondary folds of the kidney act primarily as a compensation sac for blood displaced from the cephalopedal haemocoel during retraction of the proboscis, or when the snail withdraws into its shell. Aggregates of haemocyanin form in these blood spaces at such times. The relationship between the two sets of folds, which interdigitate in pycnonephridians or are largely separate in meronephridians, is related to shell shape, proboscis type and foregut structure in different families. Loss of the anal gland in the buccinoideans is correlated with changes in the efferent renal veins, a reduction in the size of the rectal sinus and the reduction or loss of the gland of Leiblein. Extracellular bacteria occur in the anal gland of the muricid Ocenebra erinacea similar to those previously reported in Nucella, suggesting that their presence may be typically associated with the gland in the Ocenebrinae and possibly other muricids.