An Introduction to the Invertebrates, Second Edition - tiera.ru

Chapter11Mollusca: Bivalvia andCephalopodaBIVALVIA11.1 How is the molluscan body plan modifiedin bivalves?In bivalves, the molluscan body is laterally compressed betweenthe two halves of the dorsally hinged shell (Figure 11.1a).Characteristically the ctenidia are greatly enlarged, with elaborateciliary tracts used for filter-feeding. The head and its associatedsensory organs have been lost, as also has the radula, and feedingis assisted by ciliated ‘labial palps’. The foot is lifted off the ground,becoming wedge-shaped; in sedentary forms glands in the footsecrete a ‘byssus’ of threads attaching the bivalve to the substratum.Muscles are strongly developed: a pair of adductors attached tothe inner surfaces of the shell hold the two halves together, workingagainst the elastic recoil of the hinge ligament, and anterior andposterior retractor muscles hold the byssus in place. Inhalant andexhalant siphons determine the flow of water in the mantle cavity.Figure 11.1b,c,d indicates the structure of bivalves, with layersprogressively removed.11.2 What is the range of bivalves?They are very common animals, with about 8000 known species. Allare aquatic and most are marine, perhaps because the very largepermeable gill surface presents a considerable osmotic problem fora freshwater bivalve. The common mussel Mytilus extends into