Phylum Porifera: Definition, Characteristics, Classification and Examples

Phylum Porifera: Definition, Characteristics, Classification and Examples

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PHYLUM  PORIFERA

Phylum Porifera is a group of simple, multicellular, aquatic animals commonly known as sponges. They are characterized by their porous body structure, which allows water to circulate through specialized channels, enabling them to filter-feed by extracting nutrients and oxygen from the water. Most sponges are marine, though some are found in freshwater. Porifera are among the most ancient animal groups, with a simple organization that makes them pivotal to understanding the evolution of multicellular organisms.

General characteristics of Phylum Porifera

  • Multicellular organisms with cellular level of body organization. No distinct tissues or organs.
  • Mostly marine, a few freshwater (family spongillidae), all aquatic.
  • Solitary or colonial, all sessile, sedentary.
  • Body form vase-like, cylindrical, tubular, cushion-shaped, many branched, e.t.c.
  • Symmetry radial or asymmetrical.
  • The surface of the body is perforated by numerous pores, the ostia through which the water enters the body and one or more large openings the oscula by which water passes out.
  • The body of the sponges is multicellular, comprises an outer layer ectoderm (or pinacoderm), inner layer endoderm (or choanoderm) and gelatinous non-cellular intermediate layer of mesenchyme. Therefore, they are diploblastic (not truly diploblastic because cells are loosely arranged and do not form definite layers).
  • The interior space of the body is either hollow or permeated by numerous canals lined with flagellated collar cells or choanocytes. Sponges are the only metazoans having choanocytes.
  • Sponges have skeleton consisting, in some cases, of fine flexible fibres known as spongin or spongin fibres and siliceous spicules and calcareous spicules, or absent.
  • Mouth absent. Sponges feed on particles of organic matter and minute organisms ingested by the choanocytes from the currents of water passing through the body.
  • Digestion intracellular.
  • The nervous and sensory cells are absent. The body functions are carried on by the cells mostly independently or with little co-ordination.
  • Respiration takes place by the intake of oxygen which comes in with the current of water.
  • Excretion occurs by passing out the excretory products through the osculum or oscula with the current of water.
  • All sponges are hermaphrodite, but cross-fertilization is the rule.
  • Asexual reproduction by buds or gemmules.
  • Fertilization is internal and cleavage holoblastic. Development is indirect through a free-swimming ciliated larva, the amphiblastula or parenchymula which after swimming for many hours attaches itself to some substratum and develops into a small sponge.
  • The organization of sponges has been classified into three main types – ascon type, sycon type and leuconoid type.

Classification of Phylum Porifera

Note: The classification is adopted from Hyman, L.H. (1940).

CLASS 1. CALCAREA

  1. Radially symmetrical, vase-shaped bodies measuring up to 10 cm in height.
  2. Skeleton consists of calcareous spicules. Spicules are of monaxon, triaxon and tetraxon type.
  3. Choanocytes (collared flagellated cells) are large. In some cases the choanocytes line the whole spongocoel, while in others are restricted only to the radial or flagellated canals.
  4. Solitary or colonial, exclusively marine, shallow water forms.
Order 1. Homocoela
  • Homocoela includes simplest sponges.
  • Body wall is thin.
  • Spongocoel or paragastric cavity is lined by choanocytes throughout.
  • Examples: 1. Leucosolenia, 2. Clathrina.
Order 2. Heterocoela
  • Heterocoela includes big sponges with complicated structures.
  • The body wall is thick folded internally.
  • Spongocoel or paragastric cavity is lined by flattened epithelial cells.
  • Choanocytes are restricted only to the radial canals.
  • Examples: 1. Sycon or Scypha, 2. Grantia, 3. Leucilla.

CLASS 2. HEXACTINELLIDA

  1. Body cylindrical, vase or funnel-shaped, radially symmetrical, measuring up to 90 cm in length, fastened at the base by a root tuft of spicules.
  2. Skeleton composed of triaxon, six-rayed siliceous spicules.
  3. Canal system is simple having branched or unbranched radial canals.
  4. Solitary exclusively marine forms. They are mostly deep sea inhabitants and widely distributed.

Order 1. Hexasterophora

  • Spicules are hexasters, i.e., star-like in shape.
  • Radial canals or flagellated chambers are simple.
  • They are not attached by root tufts but commonly attached to a object.
  • Examples: 1. Euplectella, 2. Staurocalyptus.
Order 2. Amphidiscophora
  • The spicules are amphidiscs. No hexasters.
  • They are attached to the substratum by root tufts.
  • Examples: 1. Hyalonema, 2. Pheronema.

CLASS 3. DEMOSPONGIAE

  1. Body asymmetrical, often massive and brightly coloured.
  2. Skeleton consists of siliceous spicules or spongin fibres or a combination of siliceous spicules and spongin fibres or absent. The spicules are either straight and needle-like (monaxon) or having eight rays (tetraxon) but never six-rayed.
  3. Canal system is complicated (leuconoid type) having small spherical radial canals or flagellated chambers. Spongocoel is entirely absent.
  4. Demospongiae are widespread in nature and includes majority of present-day existing species.

Subclass I. TETRACTINELLIDA

  1. Sponges are mostly solid and simple rounded cushion-like flattened in shape usually without branches.
  2. Skeleton comprised mainly of tetraxon siliceous spicules but absent in order Myxospongida.
  3. Canal system is leuconoid type.
  4. Shallow water form.
Order 1. Myxospongida
  • Simple structure.
  • Skeleton absent.
  • Examples: 1. Oscarella, 2. Halisarca.
Order 2. Carnosa
  • Simple structure.
  • Spicules are not differentiated into megascleres and microscleres. Asters may be present.
  • Examples: 1. Plakina, 2. Chondrilla.
Order 3. Choristida
  • Spicules are differentiated into megascleres and microscleres.
  • Example: Geodia

Subclass II. MONAXONIDA

  1. Monaxonids occur in a variety of shapes from rounded mass to branching type of elongated or stalked with funnel or fan-shaped.
  2. Skeleton consists of monaxon spicules with or without spongin. Spicules are distinguished into megascleres and microscleres.
  3. They are found in abundance throughout the world.
  4. Shallow and deep-water forms.
Order 1. Hadromerina
  • Monaxon megascleres in the form of tylostyles.
  • Microscleres when present in the form of asters.
  • Spongin fibres are absent.
  • Examples: 1. Cliona, 2. Tethya, 3. Poterion, 4. Ephydatia.
Order 2. Halichondrina
  • Monaxon megascleres are often of two types, i.e., monactines and diactines.
  • Microscleres are absent.
  • Example: Halichondria
Order 3. Poecilosclerina
  • Monaxon megascleres are of two types, one type in the ectoderm and another type in the choanocyte layer.
  • Microscleres are typically chelas, sigmas and toxas.
  • Examples: 1. Cladorhiza, 2. Microciona.
Order 4. Haplosclerina
  • Monaxon megascleres are of only one type, i.e., diactinal.
  • Microscleres absent.
  • Spongin fibres are generally present.
  • Examples: 1. Chalina, 2. Pachychalina, 3. Spongilla.

Subclass III. KERATOSA

  1. Body is rounded and massive with a number of conspicuous oscula.
  2. Skeleton composed of network of spongin fibres. Siliceous spicules are present.
  3. They are also known as horny sponges, found in shallow and warm waters of tropical and subtropical regions.
  4. Examples: 1. Euspongia, 2. Hippospongia, 3. Hircinia

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