Porifera

(invertebrate zoology) The sponges, a phylum of the animal kingdom characterized by the presence of canal systems and chambers through which water is drawn in and released; tissues and organs are absent.

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 also 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. A taxonomic scheme of the Porifera follows.

Class Hexactinellida

     Subclass Amphidiscophora

          Order: Amphidiscosa

                         Hemidiscosa

                         Reticulosa

     Subclass Hexasterophora

          Order: Hexactinosa

                         Lynchniscosa

                         Lyssacinosa

Class Calcarea

     Subclass Calcinea

          Order: Clathrinida

                         Leucettida

     Subclass Calcaronea

          Order: Leucosoleniida

                         Sycettida

     Subclass Pharetronida

Class Demospongiae

     Subclass Tetractinomorpha

          Order: Homosclerophorida

                         Choristida

                         Spirophorida

                         Hadromerida

                         Axinellida

     Subclass Ceractinomorpha

          Order: Dendroceratida

                         Dictyoceratida

                         Halichondrida

                         Haplosclerida

                         Poecilosclerida

                         (Permosphincta, extinct)

Class Sclerospongiae