sponge

Any saclike simple invertebrate of the phylum Porifera, usually marine. A sponge has a hollow body, its cavity lined by cells bearing flagellae, whose whiplike movements keep water circulating, bringing in a stream of food particles. The body walls are strengthened with protein (as in the bath sponge) or small spikes of silica, or a framework of calcium carbonate.

Fossil sponges only 1 mm/0.04 in in length were found in China and described in 1998. They are estimated to be 580 million years old, the oldest discovered to date.

A deep-sea sponge found in 1994 is the first carnivorous sponge to be identified. The 15-mm/0.6-in high sponge, of the family Cladorhizidae, traps small crustaceans by entangling them in thin filaments. Epithelial cells then migrate towards the prey and envelop it.

Italian zoologists discovered algae living within some sponges in 1996. The algae receive enough light to photosynthesize via the sponge's spicules, which channel light deep into the sponge, acting in a way similar to fibre-optic cables.