Linnaeus, 1758
Laterally flattened molluscs with a shell made of two hinged valves that are connected dorsally by elastic ligaments and shell teeth; the shells are closed by internal adductor muscles.
Larval development involves a trochophore larva, followed by a veliger — the uniquely molluscan larva. The bivalve veliger larva may possess a foot and shell; the most characteristic part of the veliger is the swimming and feeding organ, called the velum, consisting of two large ciliated lobes developed from the prototroch. Some bivalves have long-lived planktotrophic veligers, whereas others have short-lived lecithotrophic veligers. Just before settlement, the veliger transforms into a still pelagic juvenile that morphologically resembles the adult.
The key recognises veligers and juveniles of bivalves, but they are not keyed out to species. See Rees, 1950 and Rees, 1951 for illustrations of juveniles of some North Sea bivalve species.
[Brusca and Brusca, 1990]