Final answer:
Mollusks have a free swimming larval stage called trochophore, which may be followed by a veliger larval stage before becoming adults. These larvae use cilia to move through the water.
Step-by-step explanation:
Mollusks go through a larval stage in which they are free swimming. The larvae are specifically known as trochophore in the early stages; these tiny organisms possess cilia, which aid in swimming. In some aquatic mollusks, another larval stage known as the veliger follows the trochophore stage, before they metamorphose into their adult form. Sexual reproduction in mollusks typically involves the release of gametes into the mantle cavity, with fertilization occurring either internally or externally, depending on the species. After fertilization, zygotes grow into trochophore larvae, which can swim freely and are characterized by ciliary bands, making them ideal for moving through the water during this stage of their development.