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

136 MOLLUSCA: BIVALVIA AND CEPHALOPODAFig.11.1 The structure of bivalves: (a) external view of zebra mussel (20 mm long); (b) diagram of mussel (Mytilus) with left shellvalve removed, showing mantle; (c) diagram of mussel with left shell valve and mantle removed, to show water currents; (d) diagramof mussel showing visceral organs (nervous system omitted for clarity); (e) Scrobicularia burrowing, showing the long siphons.estuaries, but few species live in fully fresh water. In the absenceof a powerful excretory organ, a common adaptation is dilution ofthe internal body fluid and thus reduction of the osmotic gradientacross the gills. The most dilute animal known is the freshwaterclam Anodonta, with an internal fluid concentration one-tenth ofthat of a freshwater crayfish.In size, bivalves range from a few millimetres to the giant clamTridacna which is over a metre long. Here the exposed mantle edgecontains symbiotic algae. Like those of the corals among which theclam lives, the algae may assist precipitation of calcium carbonate(needed to make the large shell) by removing carbon dioxide andreducing acidity.