How do mollusk eggs develop into trochophore larvae?

In many marine mollusks, the development of the trochophore larva begins with the fertilization of eggs, which typically undergo spiral cleavage. During this early embryonic development, the cells divide to form a structure that eventually gives rise to the free-swimming larval form.12

The trochophore is characterized by a preoral girdle of ciliated cells known as the prototroch, which enables the larva to swim. In some species, this larval stage is lecithotrophic (nonfeeding), while in others, it is planktotrophic (feeding), utilizing the ciliary band to capture microscopic organisms.314

In more advanced mollusks, such as many marine gastropods and bivalves, the trochophore larva further develops into a veliger stage. During this transition, the ciliated girdle expands to form a velum, which serves both for locomotion and feeding. As development continues, the larva eventually loses these larval structures and undergoes metamorphosis into the adult form. It is important to note that not all mollusks pass through a free-swimming larval stage; many species exhibit direct development, particularly those that are nonmarine or possess yolky eggs.31