Their bodies do not contain tissue, muscles, nerves, or organs. Some are composed of silica spicules, and others of calcium carbonate. They generally do not have body symmetry, but some sponges have radial symmetry. Their bodies are hollow and held together by jelly-like substances which have many cells. Cells on the outside can move inwards and change function. Their pores are called ostia. Common sponges have skeletons made of the protein spongin. They pump water through the body to feed, and have one body orifice to serve for ingestion as well as excretion.
Sponges are known from Precambrian deposits and representatives from all three classes range from the Cambrian to the present.
- Glass sponges (class Hexactinellida) are represented by sponges with silica spicules. Members of this class helped to build massive reefs in the late Devonian.
- Calcareous sponges (class Calcarea) are represented by sponges with calcite spicules.
- Calcarea sponges were important reef builders in the Permian and Triassic.
They are benthic and sessile, and live in marine and freshwater environments. Sponges are free-swimming in their larval stages. Sponges can have a symbiotic relationship with blue-green algae (shallow sponges are more likely to do this). Sponges can be reef builders, being able to house organisms such as nematodes, crustaceans and mollusks in the osculum (center chamber).
Fossil sponges can be used as indicators of paleoenvironments. Sponges are sensitive to currents, turbidity and depth. Thus, species of sponges can be clues to the environmental conditions present during their lives.
They were the first to branch off from the common ancestor of all animals, so they're the sister group of all other animals. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity. Some specimens can move at around 1-4 mm a day.
Although molecular clocks and biomarkers suggest sponges existed well before the Cambrian explosion of life, silica spicules like those of demosponges are absent from the fossil record until the Cambrian.
Well-preserved fossil sponges from about 580 million years ago in the Ediacaran period have been found in the Doushantuo Formation.
