Many Gastropoda deposit their eggs, after fertilization, enclosed in capsules; others, as Paludina, are viviparous; others, again, as the Zygobranchia, agree with the Lamellibranch Conchifera (the bivalves) in having simple exits for the ova without glandular walls, and therefore discharge their eggs unenclosed in capsules freely into the sea-water; such unencapsuled eggs are merely enclosed each in its own delicate chorion.
In other respects the anatomy of the cockle presents no important differences from that of a typical Lamellibranch.
Walcott has described a tiny lamellibranch (Modioloides) from the inferior Cambrian, and he reports the gastropod (?) genus Chuaria from the preCambrian.
As an example of the organization of a Lamellibranch, we shall review the structure of the common pond-mussel or swan mussel (Anodonta cygnea), comparing it with other Lamellibranchia.
It is on this account that the valves of a dead Lamellibranch always gape; the elastic ligament is no longer counteracted by the effort of the adductors.