Definition and Introduction
Lowest multicellular animals or metazoans without true tissues, i.e. , at Cellular level or body organization.
Familiar as sponges, these animals are well known for their ability to absorb and without fluids . Now a days artificial sponges are made from rubber nylon., etc, but a natural sponge is the dried skeleton of some members of this phylum, The word porifera means pore bearers (Gr. Porus= pore, ferre= to bear ), Their body wall has numerous minute pores, called ostia, through which a continuous current of outside water is drawn into the body, About 5,000 species are known .
Aristotle (384-322 B.C.) knew sponges Until 1765, these were through to be plants when ellis first observed that sponges constantly draw and pump Outside water. He therefore, recognized these animals .Due to their plant like form Linnaeus. 1758, Cuvier 1796 and Lamarck1801 included sponges, together with coelenterates, under the group Zoohyta plant like animals .Robert grant 1825 finally proved that sponges are animals , and coined the name porifera for sponges from other metazoans on basis of embryological studies, and suggested a separate group Parazoa for these.
1. All aquatic, mostly marine. Solitary or colonial, all sessile-attached to various objects in water ,mostly look like nonliving outgrowths.
2. Body shape and size variable , shape cylindrical vase shaped cup shaped, spherical or dise like. Some radially symmetrical ,but most are irregular and asym metrical in shape , white or variously coloured.
3. Body organization of cellular level i.e. true tissues, movable parts or appendages not formed . Although there is some physiological division of labour accompanied with structural differentiation amongst body cells. But similar cells are arranged neither in permanent layers nor masses to form tissues, Moreover , cell differentiation not permanent cells one kind can change intro other kinds ALso cells mostly function independently without co ordination , because there is no co ordinating nervous system. Thus sponge body is liked a mere mass of cells held together by a nonliving cementing substance .
4. Sponge architecture is constructed around a system of water canals , esta blushing a canal system .Outside water is continuously drawn into the canal system through numerous dermal pores or ostia of body wall ,collects into the main body cavity called spongocoel and then escapes out through a single large exhalent opening the osculum, situated at the free end of body .Cells obtain food and oxygen from this water current , and release CO2 and excretory substance into it. Thus, there is neither a distinct mouth, nor a digestive cavity.
5.Sponge body is diploblastic with an outer dermal ( ectodermal) layer of flattened pinacocyte cells ( pinacoderm ) and an inner gastral ( endodermal ) layer of peculiar collared cells choanocytes (choanoderm) .In between is a jelly like mesenchyme or mesohyl layer of dispersed collagen fibers. It contains some amoeboid, migratory cells of both layers.
6. A skeleton calcareous or siliceous s spicules , or of sponging fibres , or of both siliceous spicules and sponging fibres present in most sponges for supporting the body and maintaining its gross anatomy.
7. Digestion intracellular in food vacuoles as in protozoa.
8. Gaseous exchanges and excretion by diffusion through plasma membrane as in protozoa. Ammonia is called chief excretory waste.
9.No nervous system reactions largely local.
10. Reproduction both asexual and sexual. Asexual mostly by buds and gemmules .Most forms are hermaphrodite. Zygote develops into a free swimming ciliated larva.