Classes Bivalves and Cephalopods
Bivalve molluscs have no head, a body flattened from the sides and a wedge-shaped foot. The freshwater mussel lives half buried in mud on river and shallow-lake bottoms; its shell is 10–15 cm, lined inside with mother-of-pearl, closed by adductor muscles and opened by an elastic ligament. Food particles and microorganisms enter with water through the inhalant siphon and leave through the exhalant siphon; the gills take up oxygen. The mussel filters water, so it helps clean it. The sexes are separate; eggs develop in the gills, and larvae attach to fish skin as parasites. In the sea live oysters, mussels, scallops, pearl oysters that produce pearls and the giant clam, weighing up to 250 kg. Cephalopods (squid, octopus, cuttlefish) have no outer shell (in cuttlefish and squid only a small remnant remains inside the body), the front of the foot has become tentacles, and the tentacles carry suckers. Their brain is highly developed and they form complex reflexes; they move by jet propulsion, forcing water out of the funnel, and escape danger by releasing ink. They are caught for food.
Table: mussel – squid – pond snail; columns: shell, head, way of moving, breathing organ. Safety: do not touch live sea animals (especially octopus or cone snails) or shells on the shore; do not eat raw seafood.