In the veliger stage many Pectinibranchia (Purpura, Nassa, &c.) exhibit, in the dorsal region behind the head, a contractile area of the body-wall.
E, The trochosphere passing to the veliger stage, dorsal view showing the formation of the primitive shell-sac.
In development they pass through the typical trochosphere and veliger stages provided with boat-like shell.
A, Veliger-larva of an Opisthobranch (Polycera).
From the fact that Aplysia commences its life as a free-swimming veliger with a nautiloid shell not enclosed in any way by the border of the mantle, it is clear that the enclosure of the shell in the adult is a secondary process.