Many gastropod (snails and their allies) and bivalve (clams, mussels, oysters and scallops) molluscs pass through a trochophore larval stage before developing into veliger larvae.
Veligers are planktonic larvae of many bivalve and gastropod molluscs characterized by a shell, foot, and
velum (a lobed, ciliated structure used for swimming and feeding). The velum is derived from the prototroch - a pre-oral ciliated band in the trochophore larva. A dorsal shell gland secretes the shell of the veliger. The shell of a bivalve veliger is bi-valved while the shell of a gastropod veliger resembles a spiraled snail shell.
The first image shows a veliger of the marine snail
Nassarius fossatus in polarized light (hence the funny color). The spiraled shell of the veliger is in focus with the bi-lobed ciliated velum extended from the shell. A secondary (post-oral) ciliated band known as the
metatroch is also visible below the main ciliated band of the velum.
In the second image is a veliger larva of the jingle shell,
Pododesmus cepio. This larva has a bivalved shell. One valve is in focus while the other valve is located on the opposite side of the larva. The velum of this larva is pulled into the shell in this image, however its cilia can be just barely seen protruding beyond the edge of the shell.
The first image shows a veliger of the marine snail Nassarius fossatus in polarized light (hence the funny color). The spiraled shell of the veliger is in focus with the bi-lobed ciliated velum extended from the shell. A secondary (post-oral) ciliated band known as the metatroch is also visible below the main ciliated band of the velum. 
In the second image is a veliger larva of the jingle shell, Pododesmus cepio. This larva has a bivalved shell. One valve is in focus while the other valve is located on the opposite side of the larva. The velum of this larva is pulled into the shell in this image, however its cilia can be just barely seen protruding beyond the edge of the shell.
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