A well-marked trochosphere is formed by the development of an equatorial ciliated band; and subsequently, by the disproportionate growth of the lower hemisphere, the trochosphere becomes a veliger.
E, The trochosphere passing to the veliger stage, dorsal view showing the formation of the primitive shell-sac.
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.
There is a veliger stage in development, but the velum is reduced.
Except in Oncidium, there is no longer a veliger stage in development.