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.
- Embryo of Limnaeus stagnalis, at a stage when the Trochosphere is developing foot and shell-gland and becoming a Veliger, seen as a transparent object under slight pressure.
There is a veliger stage in development, but the velum is reduced.
Except in Oncidium, there is no longer a veliger stage in development.
A single otolith is present as in the veliger embryos of Opisthobranchia.
Other Lamellibranchs exhibit either a trochosphere larva which becomes a veliger differing only from the Gastropod's and Pteropod's veliger in having bilateral shell-calcifications instead of a single central one; or, like Anodonta, they may develop within the gill-plates of the mother, though without presenting such a specialized 210 1P -' 1 °* larva as the glochidium.
No ciliated An extraordinary modification of the veliger occurs in the development of Nucula and Yoldia and probably other members of the same families.
H, I, Veliger stage later than 1, The sub-pallial space, here destined to become the lung.
E, The Trochosphere passing to the Veliger stage, dorsal view showing the formation of the primitive shell-sac.
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.
As in the Rotifera, it serves the veliger larva as an organ of locomotion.
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.
The veliger, as soon as its shell has attained some extent and begins to assume definite shape, is no longer of a form common to Mollusca generally, but acquires characters peculiar to the partiY cular class to which its parents belong.
- " Veliger " embryonic form of Mollusca.
A, Earlier, and (B), later, Veliger c, Visceral dome with dependent of a Gastropod.
C, Veliger of a Pteropod showing p, Foot.