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.
(calcareous
sponges)
Features: often less than 10 cm (4 in)
high, skeletal spicules of calcium carbonate
(glass
sponges)
Features: skeletal, six-pointed, silica
(glass-like) spicules
(demosponges)
Features: some have three- or
four-pointed silica spicules
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.
