Phylum: Porifera the most primitive animals
The name porifera is derived from latin porus = pore, ferra = to bear
The porifera are pore-bearing animals, commonly called the sponges. All are aquatic. Out of total 5000 species live in fresh water while all others are marine.
These animals are composed of many cells however there is no tissue organization and have no organs. Sponges lack symmetry. In most sponges the body wall is formed of an outer layer, pinacoderm, made up of cells called pinacocytes: and an inner layer choanoderm made of flagellated collar cells called choanocytes. Between these two layers is present gelatinous mesenchyme which “may” contain arooeboid cells and spicules or sponging fibres.
Scolymastra joubini- a barrel like glass sponge of Antarctica is more than a metre tall.
The peripherals range in size from few ilimeter “wide” to more than one meter tall. They are macroscopic i.e., can be seen with naked eye. There is a single cavity inside the body, the spongocoel. In most sponges the spongocoel may be divided into flagellated chambers or canals, lined by flagellated choanocytes.
Numerous pores are present in the body wall. The pores through which water enters the body are called ostia, and pore by which the water leaves the body is known as osculum (main opening). There are no respiratory or circulatory organs.
Since the sponges are sessile, therefore these depend upon the food coming to them along with water currents brought about by movement of flagella of choanocytes. This includes small animals, (zooplankton) and plants, (phytoplankton) which constitute about 20% of their food. 80% of their food consists of detrital organic particles. The food enters the spongocoel cavity through ostia. The food is ingested by the flagellated cells, the choanocytes. The waste products either diffuse out of the sponge directly through the body wall or flow out through osculum.
The adult sponges are stationary, spending their lives attached to the rocks at the bottom or other solid objects. However, their larvae are able to move (swim).
There is no definite nervous system, however neurosensory and neuron cells are probably present which seem to coordinate the flow of water.
The skeleton is in the form of variously shaped needle-like structures called spicules. These may be calcareous or siliceous. The bath sponge has a skeleton of sponging fibres. The skeleton is present among pinacocytes and provides support. Spicules are also present around osculum and ostia.
Sponges reproduce both by asexual and sexual methods of reproduction.
The asexual reproduction in sponges is by budding. The buds may be external or internal; the internal buds are called gemmules. Both types of buds develop into new sponges.
Some sponge species reproduce sexually. These are mostly hermaphrodite, mostly protandrous, i.e. male sex cells develop first. In some sponges the sexes are separate.sperms released in water are carried to the eggs by amoeboid cells. Fertilization occurs in mesenchyme and zygote is formed. The embryo development includes blastula and larval stages.
Examples of sponges are:
- Sycon: it is typical marine sponge.
- Leucoselenia: a sponge that consists of group of erect tubes.
- Euplectella: it is beautiful and delicate sponge made up of glassy framework. It is
Commonly called Venus flower basket.
4. Spongilla: it is freshwater sponge.
The skeleton of sponges have long been used by man mostly for washing and bathing. Although many artificial spnges have been made from synthetic material, still the natural sponges are in demand and is an important industry in many parts of the world. The best commercial spnges are found in the warm waters of Mediterranean Sea. Spnges have grat capacity to absorb water. They are used in surgical operations for absorbing fluids and blood. They are also used for sound absorption in buildings.