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Porifera (pōrĭfˈərə) [Lat.,=pore bearer], animal phylum consisting of the organisms commonly called sponges. It is the only phylum of the animal subkingdom Parazoa and represents the least evolutionarily advanced group of the animal kingdom. All adult sponges are sessile (nonmotile), and nearly all are marine; there are six families of freshwater sponges. Sponges are subdivided into three classes.
Sponges lack organs and tissue, and all the cells exhibit considerable independence. The sponge is made up of two single-cell-deep layers and an intermediate mesohyl (mobile cells plus extracellular matrix). The outer (sac) layer consists of flattened polygonal cells called pinacocytes. The middle (mesohyl) layer consists of gelatinous protein/carbohydrate material, a range of mobile cells, and a skeleton of calcareous or siliceous spicules, or of elastic proteinaceous fibers called spongin fibers. The inner layer consists of flagelled cells called collar cells, or choanocytes.
The body is permeated by numerous pores called ostia that open into inhalant canals that lead to the feeding chambers, which are made up of choanocytes; here also are large openings, termed oscules, fed by exhalant canals, that carry the water current from the choanocyte chambers to the exterior. The concerted whipping action of the choanocyte flagella creates a current of water from ostia through the sponge body oscules. The choanocytes filter plankton and small bits of organic detritus from the water and, like the pinacocytes, absorb oxygen. Food is digested in ameboid archaeocytes that pick up food vacuoles from the choanocytes, which ingest the mainly particulate food. Waste products are carried out through the osculum.
Different types of amoebocyte spongiocytes and sclerocytes are responsible for secreting the skeletal material. Achaeocytes give rise to egg cells and sperm derive from choanocytes. The body of most sponges is irregular in form, although an almost radial symmetry is displayed by some. Three types of sponge structure are recognized: the asconoid, the most primitive, is regular, tube-shaped, and radially symmetrical; the syconoid is a more irregular structure that displays some degree of folding of the body wall while still maintaining a basic radial symmetry; the leuconoid is highly irregular, displays the greatest degree of folding of the body wall, and has lost radial symmetry. In the leuconoid sponges choanocytes line the pockets formed by the convoluted body wall.
Sponges are limited in size by the rate at which water can flow in and out of the spongocoel, bringing in food-bearing water and oxygen and removing waste products. Because the asconoid type has the smallest surface area, sponges of this structure are among the smallest in the phylum; leuconoid sponges, with a large amount of surface area, represent some of the largest members of the phylum.
Class Calcarea (Calcareous Sponges)
Class Hexactinellida (Glass Sponges)
Class Demospongiae (Demosponges)
A name proposed for a subkingdom of animals which includes the sponges. Erection of a separate subkingdom for the sponges implies that they originated from protozoan ancestors independently of all other Metazoa. This theory is supported by the uniqueness of the sponge body plan and by peculiarities of fertilization and development. Much importance is given to the fact that during the development of sponges with parenchymella larvae, the flagellated external cells of the larva take up an internal position as choanocytes after metamorphosis, whereas the epidermal and mesenchymal cells arise from what was an internal mass of cells in the larva. These facts suggest that either the germ layers of sponges are reversed in comparison with those of other Metazoa or the choanocytes cannot be homologized with the endoderm of other animals. Either interpretation supports the wide separation of sponges from all other Metazoa to form the subkingdom Parazoa or Enantiozoa. On the other hand, there are cogent arguments in favor of the basic similarity of the development of sponges and other Metazoa. See Metazoa, Porifera