Except in Oncidium, there is no longer a veliger stage in development.
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.
A, pilidium larva anus by the closure in of nemertine; B, Asplanchnapus the middle of such a slitschematized; C, a ploimal rotifer; like blastopore opening D, trochosphaera female (schematized into a sack-like stomach from Semper); E, veliger larva of mol as is seen in the larvae lusc; F, trochophore larva of annelid.
The development of Aplysia from the egg presents many points of interest from the point of view of comparative embryology, but in relation to the morphology of the Opisthobranchia it is sufficient to point to the occurrence of a trochosphere and a veliger stage (fig.
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.
In development they pass through the typical trochosphere and veliger stages provided with boat-like shell.
It now passes to the veliger phase, a definitely molluscan form, in which the disproportion between the area in front of the ciliated circlet and that behind it is very greatly increased, so that the former is now simply an emarginated region of the head fringed with cilia.
The transition from the free-swimming veliger larva with its nautiloid shell (fig.
No ciliated An extraordinary modification of the veliger occurs in the development of Nucula and Yoldia and probably other members of the same families.
The body of the veliger is characterized by the development of the visceral hump on one surface, and by that of the foot on the other.