IV.

Reproduction

The basic mollusk pattern is to have separate sexes, with sperm and eggs spawned into the water, where fertilization and early development occur. In most mollusks a larval stage follows, in which the larvae swim about for a while and then settle on the bottom and mature; this stage is often modified or absent, however. Fertilization may also be internal, with glands secreting protective coverings around the eggs. Slow-moving creatures such as snails often evolve into hermaphrodites (both male and female), because this doubles the number of appropriate mates. Sometimes the mother protects the developing eggs. Some oysters are remarkable in caring for the young inside the mantle cavity and switching back and forth from being males to being females.