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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)
The sponges, a phylum of the animal kingdom which includes about 5000 described species. The body plan of sponges is unique among animals. Currents of water are drawn through small pores, or ostia, in the sponge body and leave by way of larger openings called oscula. The beating of flagella on collar cells or choanocytes, localized in chambers on the interior of the sponge, maintains the water current. Support for the sponge tissues is provided by calcareous or siliceous spicules, or by organic fibers, or by a combination of organic fibers and siliceous spicules. Some species have a compound skeleton of organic fibers, siliceous spicules, and a basal mass of aragonite or calcite. The skeletons of species with supporting networks of organic fibers have long been used for bathing and cleaning purposes. Because of their primitive organization, sponges are of interest to zoologists as an aid in understanding the origin of multicellular animals. See Animal kingdom, Parazoa
The Porifera have a fossil record extending from the Precambrian to Recent times. More than 1000 genera of fossil sponges have been described from the Paleozoic, Mesozoic, and Cenozoic eras. The living Porifera are divided into four classes on the basis of their skeletal structures: Hexactinellida; Calcarea; Demospongiae; and Sclerospongiae.
a phylum of invertebrate animals that consists of a single class, Spongia, which is represented by four orders—Calcarea (calcareous sponges), Hexactinellida (siliceous), Tetractinellida, Cornacuspongida.
Porifera are the most primitive multicellular animals, having no clearly differentiated tissues or organs. The body of the typical sponge is either goblet-shaped or cylindrical, attached at its base to a substrate; at its free end there is a wide aperture called the osculum, which communicates with the atrial, or paragastral cavity inside the sponge. Scattered over the surface are numerous pores, which lead into canals that penetrate the body walls. All sponges are diploblastic; the outer layer is formed of squamous epithelium and the inner layer consists of collar cells (choanocytes). Between them lies an interlayer of unstructured gelatinous matter, or mesoglea, which contains various types of cells, including amoebocytes, collencytes, and scleroblasts. Three types of sponge structure are distinguished, depending on the degree of development of the canal system and the localization of the choanocytes: ascon, sycon, and leucon. In the ascon type, the choanocytes line the atrial cavity and the thin body walls are perforated by simple foraminal tubules. In the sycon and leucon types the choanocytes are concentrated within the body walls themselves in special flagellate canals or chambers. These chambers communicate with the external pores by means of incurrent canals and open either immediately into the atrial cavity (sycon type) or are connected with it by a system of efferent canals (leucon type). Almost all sponges have a skeleton, which is most often formed of spines, or spicules (monaxons or triradiate, quadriradiate, and hexactinal spiculues and their derivatives), which consist of silica or calcium carbonate; more rarely (in Keratosa, or keratose sponges), the skeleton is represented by fibers of an organic matter called spongin.
There are 5,000 species of porifera; all are aquatic, predominantly marine, animals that lead a sessile, nonmotile life. Sponges are found in coastal waters and out to depths as great as 8,000–8,500 m. In the northern and far eastern seas of the USSR there are over 300 species; in the Black Sea, about 30; and in the Caspian, one. Freshwater sponges are represented in the USSR by the Baikal sponge and several species of Spongillidae.
The life processes of Porifera are connected with the continual filtration of the surrounding water, which, as a result of the coordinated beating of the flagella of the numerous choanocytes, enters the pores and, passing through the system of canals and the atrial cavity, flows out again through the mouth; tiny food particles (detritus, protozoans, diatoma-ceous algae, bacteria) enter the sponge, just as metabolic products are eliminated, with this water. The food is trapped by the cells of the walls of the incurrent canals or choanocytes.
The majority of Porifera are hermaphrodites. The germ cells develop in the mesoglea. The spermatozoa leave the sponge body, penetrate other individuals with mature ova, and fertilize them. A ciliate larva (planula or amphiblastula) develops from the egg. This then flows out, swims in the water for a time, and, settling finally on the bottom, is transformed into a young sponge. During the metamorphosis a process is observed that is unique to Porifera—the so-called inversion of germ layers, in which the cells of the outer layer migrate to the inside, while those inside move to the surface. Various forms of asexual reproduction, such as budding and gemmule formation, are common in sponges. Buds that grow out of the body of the sponge usually do not separate from the maternal organism; this leads to the formation of colonies of the most varied forms, such as in the shape of little trees or bushes spread across a substrate of bark, or of massive laciniate tubers.
The height of sponges varies from a few millimeters to 1.5 m and more. Their practical value is not great. Certain corneosiliceous sponges have commercial value as toilet articles and for medical and technical purposes, and the skeletons of siliceous sponges are used as ornaments.
The most ancient remains of Porifera are the spines found in Precambrian deposits. The Paleozoic era is characterized by only 15 families of Porifera; several families arose in the Paleozoic and became extinct during the Mesozoic, during which more than 60 new families both appeared and died out. There have been about 30 families since the Mesozoic, ten of which appeared during the Paleogene. The skeletal remains of Porifera sometimes form “sponge strata.” There are rocks called spongioliths, which have been enriched with the silicon from spicules.
REFERENCESRukovodstvo po zoologii, vol. 1. Moscow-Leningrad, 1937.
Osnovy paleontologii: Gubki, arkheotsiaty, kishechnopolostnye, chervi. Moscow, 1962.
Zhizn’ Zhivotnykh, vol. 1. Moscow, 1968.
V. M. KOLTUN