in Habitat

Venusta, Sandy Hook Is. Esperance, Australia (photo Peter Clarkson)

It's the animals foot, which it uses to suck onto the sponge. When you touch a shell on a sponge it will retract the foot and let go of the sponge in all cases unless sitting on eggs and then it holds fast. The grey sponge, the Venusta is sitting in is a typical venusta venusta sponge and their favourite food.