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Phylum Porifera: General Characters and more
Sponges constitute the Phylum Porifera ( L. porus, pore+ ferre, to bear) which are the simplest type of metazoans. The term Porifera was coined by Robert Grant and it means “pore bearers” due to the presence of tiny pores on their body. The study of Porifera or sponges is called Parazoology. The Phylum Porifera include more than 5000 animal species, mostly marine, but includes about 150 fresh water inhabitants also. They found in all seas, wherever there are rocks, shells, submerged timbers, or coral to provide a suitable substratum. Most sponges prefer relatively shallow water, but some groups including most glass sponges, live in deep water.
- They are the most primitive metazoans.
- Sponges are multicellular animals lacking specialized tissues and organs.
- Body grows to various shapes. Most of them are asymmetrical and some are radially symmetrical.
- They are diploblastic animals[ (Having two germ layers ectoderm(pinacoderm) and endoderm(chanoderm)].
- They are simple animals having cellular level of organization. Cells are not organised to tissues or organs.
- The bodies of the sponges are supported by spicules made up of calcium, silica or sponging fibres.
- The name Porifera means pore bearing animals. The body wall has numerous pores called ostia. The body encloses large cavity called spongocoel. Water enters the spongocoel through the ostia and inter connecting canals present in the body wall. All inter connecting are collectively called canal system. Water leaves the spongocoel through a large aperture called osculam. The canals inside the body are linked by flagellated collar cells or choanocytes. The food particles present in water current are taken by these cells.
- They lack a metazoan type of digestive system. They rely entirely on intracellular digestion.
- Mouth, digestive cavity and anus are absent.
- Nervous system and sense organs are also absent.
- Reproduction is both by sexual or asexual means.
- Asexual reproduction takes place by budding or by specialized cell called gemmules. Gemmules develop into new sponges during favourable conditions.
- Life cycle includes ciliated free swimming larvae called amphiblastula or parenchymula.
- Sponges show greater power of regeration.
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Classes of Phylum Porifera
Phlum Porifera include three classes, chiefly based on the type of its skeleton. The skeleton of some sponges are utilised for many purposes such as washing, bathing, scrubbing, mopping etc.
1. Class- Calcarea:These include the sponges with skeleton of calcareous spicules.
Example: Leucosolenia, Sycon
2. Class Hexactinellida:They include glass sponges which are most symmetrical.
3. Class Demospongia: Largest class of sponges with brilliant coloration .Skeleton formed of siliceous spicules and sponging fibres.
Example: Euspongia (Bath sponge), Spongilla (Fresh water sponge), Thena, Ciliona