The Phylum Porifera are multicellular organisms which have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl contractted between two thin layers of cells. Sponges have unspecialized cells that can transform into other types and which often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food, oxygen and remove wastes.Some sponges also deposit exoskeletons that lie completely outside their organic components. For example sclerosponges("hard sponges") have huge calcium carbonate exoskeletons. These exoskeletons are deposited by the pinacocytes which form the animals' skins.
- All sponges in this class are strictly marine, and, while they are distributed worldwide, most are found in shallow tropical waters.
All three sponge body plans are represented within class Calcarea : asconoid, syconoid, and leuconoid. Typically, calcareous sponges are small, measuring less than 10 centimeters in height, and differ in color.
Calcareous sponges vary from symmetrical vase-shaped body types to colonies made up of a mix up of thin tubes. The skeleton has either a mix up or honeycomb structure.
The Demospongiae are the largest class in the phylum Porifera. Their skeletons are made of spike-like structures consisting of fibers of the protein spongin and the mineral silica. Where the spike-like structures of silica are present, they have a different shape from those in the similar glass sponges. The demosponges include 90% of all species of sponges.
There are many diverse orders in this class, including all of the large sponges. Most are marine dwellers, but several live in freshwater environments. Some species are brightly colored, and there is great variety in body shape.The largest species are over 1 metre (3.3 ft) across. They reproduce both sexually and asexually.
Sclerosponges are sponges with a soft body that covers a hard, skeleton made of calcium carbonate. Because of their long life span (500-1,000 years) it is thought that analysis of the aragonite skeletons of these sponges could extend data regarding ocean temperature, salinity, and other data farther into the past than has been previously possible.
Their dense skeletons are deposited in an organized chronological manner, or in bands. The layered skeletons look similar to reef corals. Therefore sclerosponges are also called coralline sponges.