The Cambridge natural history, Vol. 03 (of 10)

part in front of the mouth, the larger bearing the mouth and anus.

So far the series of changes undergone by the embryo are not peculiar to the Mollusca; we now come to those which are definitely characteristic of that group. The stage next succeeding the development of the trochosphere is the definitive formation of the _velum_, a process especially characteristic of the Gasteropod...