Functional aspects of gastropod mucus
The gross anatomy and histochemical properties of the mucus-producing glands of three prosobranchs from different habitats (Littorina littorea, Bithynia tentaculata and Pomatias elegans), and a pulmonate slug (Limax pseudoflavus) were investigated. A supra-pedal gland was present in Pomatias and Limax that secreted mucoprotein and acid mucopolysaccharide in the former and neutral mucopolysaccharide in the latter. All species examined except Pomatias possessed glandular areas at the leading edge of the foot secreting a mucoprotein, and widely distributed glands on the ventral surface of the foot secreting a variety of mucosubstances, The ventral secretory cells of Pomatias, which contained protein and both neutral and acid mucopolysaccharides, were confined to a band either side of a deep medial pedal furrow. The secretory cells on the exposed dorsal surface of all four species contained a variety of mucosubstances and protein. However, a neutral mucopolysaccharide component was found to be absent in the dorsal body mucus of Limax. In the prosobranchs examined, specialized cells of the opercular groove and disc produced mucopolysaccharide and at least two differently staining proteins. In Pomatias and Bithynia the dorsal mantle epithelium possessed no secretory cells whereas that of Littorina possessed cells secreting mucopolysaccharide. At the mantle edge of Pomatias and Littorina two types of glands produced a variety of mucosubstances but at that of Bithynia one cell type produced mainly protein. The hypobranchial gland of Pomatias contained no specialised cell types but that of Bithynia and Littorina possessed cells secreting mainly protein and acid mucopolysaccharide respectively. The mode and mechanism of discharge of the mucus-producing gland cells of these gastropods and functional aspects of the distribution of cell types and their mucus are discussed. It is suggested that the pedal mucus of Limax is especially suitable for the gastropod creeping system. Many areas of the body surface of Limax and the foot of Pomatias, Bithynia and Littorina are capable of producing both fluid (neutral or weakly acid) and viscous (acid) mucus. It is postulated that such an arrangement allows for both adhesion and lubrication at different times.