How does a trochophore larva develop into a veliger larva?

The development of a trochophore larva into a veliger larva is a key transition in the life cycle of many marine mollusks, including gastropods.123

The process is primarily driven by the expansion of the trochophore's ciliary girdle, known as the prototroch. As this girdle widens, it transforms into large, heavily ciliated lobes called the velum. This structure serves multiple vital functions for the veliger, including swimming, gas exchange, and the entrapment of food particles from the water column.143

In gastropods specifically, this developmental stage is also marked by torsion, a unique 180° twisting of the body that repositions the posterior part of the body to an anterior position behind the head. As the veliger continues to develop, other essential anatomical features begin to appear, such as the shell, mantle cavity, tentacles, and foot. Eventually, the larva loses the velum and undergoes metamorphosis to transition into its adult form.413