Final answer:
The motile portion of the sponge life cycle is the larval stage, where the free-swimming larvae use cilia to move through the water before settling down to form sessile adults.
Step-by-step explanation:
The portion of the sponge life cycle that is motile is the larval stage. During the sexual reproduction of sponges, spermatozoa, which arise from choanocytes, are released into the water through the osculum and carried by water currents to fertilize the oocytes. The resulting zygote develops into a free-swimming larva. This larva, unlike the adult sponge, is motile, moving through the water using its cilia. Eventually, as the larva grows, it will settle down to form a sessile adult sponge, losing its motility in this final stage of its life cycle.
Adult sponges are sessile, meaning they do not move and are permanently attached to a fixed substrate. However, in the larval stage, sponges have the capability for locomotion, which is the only time in their life cycle when they are motile. While sponge cells are capable of creeping movements, these are localized to microenvironmental adjustment rather than overall locomotion.