DK Science: Sponges
The simplest of all animals, most sponges live in colonies (groups) that are little more than units of cells organized into two layers. Most live in the sea and are usually hermaphroditic ? each sponge produces both eggs and sperm. The larvae are free-living, but adults are sessile ? they remain anchored in one place.
Sponges have a skeleton of spicules (pointed structures) but no distinct body parts. Many are essentially a tube, closed at one end. They are not symmetrical. There are about 10,000 species.
Features: often less than 10 cm (4 in) high, skeletal spicules of calcium carbonate
Sponges are classified by their spicules, the pointed structures that make up a sponge?s framework. In a calcareous sponge these are made of calcium. There are about 150 species of calcareous sponge.
Tube sponges, or demosponges, are supported by a framework of spongin, a material similar to keratin, the substance in our fingernails. They filter food from water drawn in through pores in the colony wall. The water exits through an opening called an osculum. Special cells called collar cells help to keep the water flowing.