Porifera are also known as pore bearing organisms. They are also known as sponges. The sponges were recognized by Ellis and Aristotle was the first one to identify it. They were included as animals and were given the name porifera by Robert Grant. Sometimes they were placed in the different group known as Parazoa.
All sponges live in water. They are mostly marine and rarely occur in fresh water. They may occur alone or in colonies. They are attached to the substratum. They have different size and shapes. They are cylindrical, spherical or cup shape etc. They have pores which are of two types known as ostia and oscula. Ostia are inhalant pores while oscula are exhalent pores. They do not have symmetry and are derived from two germ layers which are known as ectoderm and endoderm respectively. So they are diploblastic animals which are multi cellular. Their level of organization is cellular and their body wall is multi layered. It consists of dermal layer, gastral layer and mesenchyme. Dermal layer is also known as pinacoderm which is the outer layer. It consists of pinacocytes and porocytes. Gastral layer is also known as choanoderm. It is the inner layer which consists of choanocytes which are flagellates. Choanocytes are also known as collar cells. Basement membrane is absent in both of the layers. Mesenchyme is also known as mesohyl layer. It is the layer which does not contain cells. This layer occurs between gastral layer and dermal layer. It has spongy fibres and has amoeba like cells known as amoebocytes. They are of different types like archaeocytes (which are undifferentiated totipotent cells that can form other cells), trophocytes (act as a food for developing cells (act as a food granules), gland cells(secrete a slimy substance), collencytes(secrete sponging fibre), scleroblasts (secrete spicules and calcareous cells), myocytes (help in the closing and opening of osculum cells), germ cells(act as a sperm and ova in the fertilizing period), chromocytes (act as a pigment granule cells) and phagocytes (act as a centre for food collection with the ingestion of damaged tissue and excreta).
The sponges have quite complex structure of pores and canals referred as canal system. It includes different types of canal systems like ascon, sycon and leucon. Ascon is the simple type of canal system. Sycon is the complex form of canal system. Leucon is the most complex form of canal system.
Spongocoel is the central body cavity of a sponge. The water flow through canal system is very vital for the survival of sponge. As it brings food and oxygen and removes the carbon dioxide and nitrogenous wastes. So it helps in all the vital processes for living.
The sponges have an internal skeleton which is made up of calcium or silicon materials. Their digestion is of intracellular type with the circulation of the food done by amoebocytes. They respire through the plasma membrane of cell. They excrete their wastes by plasma membrane and have ammonia as their main excretory product. Their mode of reproduction is both sexual as well as asexual. Asexual reproduction occurs by budding and gemmules. The gemmule consists of multiple archaeocytes. They are covered by membrane which protects them from adverse conditions. When the conditions are reversed the archaeocytes come out and form sponge. The gemmule form of reproduction is also known as internal budding. They have the higher power of regeneration. They are hermaphrodite and have the internal method of fertilization. A zygote is formed after fertilization. The zygote divides by holoblastic cleavage which means the complete division of zygote. Their development is not direct and includes larva which are free swimming.
The sponges are better than protista in many aspects. They are multi cellular and have a large body. The work is divided between the cells and has male and female gametes which are known as sperm and ova respectively. The zygote divides and forms embryo.
