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expalin what is porifera ?

Porifera are commonly referred to as sponges.  An early evolutionary branching event in the history of animals may have separated the sponges from other metazoans, and from there the sponges seem to have traveled their own separate evolutionary route.  Therefore, they are frequently placed in their own subkingdom, Parazoa, meaning "beside the animals".  Until the nineteenth century, sponges were considered a plant-animal, because during their adult life they are all sessile (attached to something else, like a rock; not free to move about).

    Fossil sponges are among the oldest known animal fossils.  The living sponge species are classified in the phylum Porifera, which is composed of three distinct groups, the Hexactinellida (glass sponges), the Demospongia, and the Calcarea (calcareous sponges). There is a fourth, small group, known as Sclerospongiae, which has the characteristics of all three of the other groups combined, but it is generally not mentioned as a major group. Sponges can be found on ocean floors in most places around the world.  Most live along the coast in shallow water, but some, such as the fragile glass sponge, are found very deep in the ocean, where water currents are very slow and do not disturb them.  A few sponges can even be found in fresh water.

    There are approximately 5,000 living species of sponges.  A sponge is essentially a water-filtering system.  The sponges don't move and all species must feed on food particles suspended in the water. In all cases, poriferans have a canal system, through which they pump water. Water enters through pores called ostia, flows through canals to a spacious chamber called a spongocoel, and finally exits through large openings called oscula. Sponges have no specialized reproductive, digestive, respiratory, sensory, or excretory organs found in this group.  Most often, they are asymmetrical, amorphous creatures, although some spectacular basket and vase-shaped species do occur.


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