This is a trochophore larva of an unidentified gastropod mollusk collected from the plankton on February 1st 2012 off a dock in Charleston. It swims about within its remarkable bilayered egg capsule. What appears as an opening at about one o’clock is most likely the micropyle (a remnant of the oocyte’s attachment to the ovary, which is also likely the site of sperm entry). The trochophore is characterized by the presence of the prototroch - a pre-oral ciliated band, which you can clearly see in this picture. Some mollusks with intracapsular development (as opposed to pelagic development) may have a reduced trochophore stage or no trochophore stage at all. Also, the stage at which the larva hatches out of its capsule can vary from species to species: some may hatch out as trochophores, while others hatch out as veligers, as this one did.
Molluscan veliger larvae are characterized by the presence of a shell and a velum whose ciliated lobes are used for locomotion and food capture. The velum is derived from the prototroch. The picture on the left shows a large bilobed velum of a different gastropod veliger.