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.
The nautiloid shell In Planorbis and in Auricula (Pulmonata, formed on the larva allied to Limnaeus) the olfactory organ is being cast, and a new on the left side and receives its nerve from shell of a different form the left visceral:ganglion.
Aplysia, like other Mollusca, develops a primitive shellsac in its trochosphere stage of development, which disappears and is succeeded by a nautiloid shell (fig.
In the larva a nautiloid shell is developed which is coiled exogastrically, that is, dorsally, and the pallial cavity is posterior or ventral (fig.
A shell is possessed in the adult state by but few Opisthobranchia, but all pass through a veliger larval stage with a nautiloid shell (fig.
The transition from the free-swimming veliger larva with its nautiloid shell (fig.
He referred also to the nautiloid shell of the larva falling to one side.