Porifera : Characteristics and Classification

Scottish Anatomist and Zoologist Robert Edmond Grant proposed the name Porifera (L. Porus, pore and ferre, to bear). The representatives of Phylum Porifera are also known as Sponges. They are simple types of aquatic invertebrates. This phylum contains about 10000 species; among them, most of the species are Marine and some are freshwater (about 100 species). Their body is asymmetrical or radially symmetrical with a variety of sizes and colors. They can prefer to live in tidal zones to almost 9,000-meter depth ranges. Most of the sponges are attached to submerged hard rocks.

General characteristics of Porifera

image of labelled diagram of Scypha
Fig. Labelled diagram of Scypha (=Sycon)

  • They are commonly known as sponges.
  • The shape of the body is cylindrical, vase-like or rounded.
  • Body is radially symmetrical or asymmetrical with the variable shape and sizes.
  • They are sedentary aquatic animals, mostly marine but a few are fresh water.
  • They may occur singly or colonial.
  • They are multicellular animals and their body bear two layers (diploblastic animals), of which an outer layer is known as pinacoderm while an inner layer is known choanoderm.
  • Between pinacoderm and choanoderm, mesophyll or mesenchyme is present which contains various kinds of amoeba-shaped cells or amoebocytes.
  • Body surface bears numerous pores, known as ostia (singular, ostium) through which water enters in the body via canal system into the central body cavity or spongocoel.
  • At the top of the body, a single large opening is present which is known as osculum.
  • They bear a canal system which helps to intake water current through the body.
  • They have no mouth, digestive cavity, and sense organs.
  • Intracellular digestion occurs in their body and their nutrition type is holozoic.
  • They have no excretory and respiratory organs and in this case, they use body surface for excretion and respiration.
  • The body consists of different types of spicules such as spongin fibers or calcareous/siliceous spicules.  
  • They feed on organic particles and minute organisms using water current via the canal system and are ingested by the flagellated cells or choanocytes.
  • Both sexual and asexual reproductions occur; Asexual reproduction occurs by forming external or internal budding
  • Fertilization is external and development indirect through two types of larvae such as amphibalstula or rhagon larva.
  • They have powerful regeneration capacity.
image of canal system
Fig. Canal system of Porifera

image of spicules
Fig. Different types of Spicules

Classification of Phylum Porifera

Phylum Porifera is divided into the following four classes on the basis of the skeleton system.

  • Class 1: Calcarea
  • Class 2: Hexactinellida
  • Class 3: Demospongiae
  • Class 4: Sclerospongiae
imager of Classification of Porifera
Fig. Classification of the Phylum Porifera at a glance

Class 1: Calcarea

  • They are exclusively marine animals.
  • They can live solitary or colonial.
  • Body shape is cylindrical.
  • The body bears large and conspicuous choanocytes cells.
  • Body is radially symmetry.
  • The exoskeleton is formed with calcareous spicules(calcium carbonate).

The class Calcarea is divided into the following two  orders:

Order-1: Homocoela

  • They are asconoid sponges with small radially symmetrical cylindrical bodies.
  • They have thin body wall which is not folded internally.
  • The body cavity or spongoceel is lined throughout the collar cells or choanocytes.

Examples:Clathrina clathrus,  Ascute asconoides, Leucosolenia cervicornis, Dendya clathrata.

Order-2: Heterocoela

  • They are syconoid or leuconoid type sponges(Canal system: Leuconoid or Syconoid) with com­paratively large bodies.
  • They have vase shaped body.
  • Choanocytes are restricted within radial canals only.
  • The body wall is thick which is folded internally.
  • Spongocoel is lined by epithelial cells.

Examples: Scypha gelatinosum, Leucosolenia cervicornis

image of Scypha
Fig. Scypha

image of Leucosolenia
Fig. Leucosolenia

Class 2: Hexactinellida

  • They are exclusively marine animals.
  • Body is more or less cup-shaped with radial symmetry.
  • They are pale in color and can grow up to 10-30 cm in height.
  • They are also called glass sponges.
  • The body bears branched or un-branched canal system.
  • The exoskeleton is composed of six-rayed siliceous spicules.
  • They can live solitary.

Examples: Hyalonemma, Pheronema,  Euplectella,

image of Hyalonema
Fig. Hyalonema

Image of  Pheronema
Fig. Pheronema

The class Hexactinellida includes the two orders:

Order-1: Hexasterophora

  • Endoskeleton is composed of hexasters type of spicules.
  • They never contain amphidiscs type spicules.
  • They have simple type radial canals or flagellated chambers which lie radially in the sponge wall.
  • They are attached to hard substratum.

Examples:Euplectellaaspergillum (Venus’s flower basket)

image of Euplectella aspergillum
Fig. Euplectella aspergillum (Venus’s flower basket)

Order-2: Amphidiscophora

  • The endoskeleton consists of amphidisc types spicules, that have a stellate disk at each end.
  • They do not contain hexaster types of spicules.
  • They are attached to the substratum using root tufts. 

Examples:Hyalonema aculeatum (Glass rope sponge), Pheronema annae (Bowl sponge).

Class 3: Demospongiae

  • Most of the species are marine and some are freshwater.
  • They have irregular body shape and asymmetrical.
  • The body bears a complicated type of canal system(Rhagon type).
  • Body cavity or spongocoel is absent.
  • This class includes nearly 8,800 species.
  • The exoskeleton is composed of siliceous or spongin fibers or both or none.

Examples: Spongilla, Euspongia

image of spongilla
Fig. Spongilla

image of Euspongia
Fig. Euspongia

The class Demospongiae is divided into the following three subclasses:

Subclass-1: Tetractinellida

  • The endoskeleton consists of tetraxon type silicious spicules or without spicules.
  • They possess leuconoid type canal system.
  • They have solid rounded or flattened body without branches.
  • They occur in the shallow water region of the sea.

Subclass Tetractinellida is divided into the following three orders:

Order-1: Myxospongida

  • They have simple shaped body. 
  • The body does not contain endoskeleton or spicules.

Examples: Oscarella carmela, Halisarca australiensis.

Order-2:  Carnosa

  • The endoskeleton contains similar sized spicules.
  • Microscleres and megascleres are not definitely differentiated.
  • Asters may be present.

Examples: Plakina monolopha, Plakortis dariae.

Order-3: Choristida

  • The endoskeleton is consists of megascleres and microscleres.
  • Spicules are differentiated into megascleres and microscleres.

Examples:  Geodia barretti,  Craniella elegans

Subclass-2:  Monaxonida

  • They have variety of body shapes which may be rounded or elongated or stalked with funnel or fan shaped.
  • The endoskeleton consists of monaxon spicules with or without spongin fibers.
  • They are distributed throughout the world.
  • They inhabit shallow or deep water region of the sea.

The subclass Monaxonida includes the following four orders:

Order-1: Hadromerida

  • Monaxon megascleres spicules present in the form of tylostyles. In this case, the broad end is knobed.
  • Generally, microscleres spicules are absent when present, they are found in the form of asters.
  • Spongin fibres are absent.

Examples: Tethya aurantium (the golf ball sponge or orange puffball sponge) , Cliona patera (Neptune’s cup sponge), Cliona celata (Bor­ing sponge).

image of Tethya aurantium

Fig. Tethya aurantium (the golf ball sponge or orange puffball sponge) 

Order 2. Halichondrida

  • The endoskeleton is composed of monaxon megascleres spicules. In this case, monaxon megascleres are often of two types, such as monactins and diactins.
  • Microsclers are usually absent.
  • Spongin fibers are present but insufficient.

Example:Halichondria panicea (breadcrumb sponge)

Order 3. Poecilosclerida

  • Monaxon megascleres are present, among them, some found  in the ectoderm and another type in the choanocyte layer.
  • Microsclers spicules are typically found in the form of chelas, sigmas and toxas.

Examples:Myxilla incrustans, Cladorhiza tenuisigma, Clathria atrasanguinea

Order 4. Haplosclerida

  • Monaxon megascleres are present in the form of diactinal. In this case, growth of the spicules occur at both ends.
  • Microscleres spicules are absent.
  • Spomgin fibres are present.

Examples:Haliclona oculata (eyed finger sponge), Spongilla lacustris (freshwater sponge) , Ephydatia fluviatilis (Freshwater sponge).

image of Haliclona oculata

Fig. Haliclona oculata (eyed finger sponge)

Subclass-3. Keratosa

  • They have rounded and massive body with a number of conspicuous oscula.
  • The endoskeleton is composed of network of spongin fibres only.
  • They do not contain silicious spicules.

Examples: Spongia catarinensis, Spongia officinalis (Bath sponge),

Hippospongia communis (horse sponge),  Phyllospongia papyracea.

image of Spongia officinalis (Bath sponge

Fig. Spongia officinalis (Bath sponge)

Class-4: Sclerospongiae

Hartman and Goreau (1970) proposed another class Sclerospongiae under the Phylum Porifera but the recent studies suggest that the class ‘Sclerospongiae’ should be placed under the class ‘Demospongiae.

  • They are leuconoid sponges and their endoskeleton is composed of calcium carbonate, silica and spongin fibres.
  • They occur in deep water of the sea.

Examples:Astrosclera willeyana, Stromcitospongia, Hispidopetra miniana.

Concluding Remarks

The term ‘Porifera’ is derived from two Latin words ‘Porus’ meaning pores and ‘Ferre’ meaning bear. They are predominantly marine organisms which inhabit in the intertidal to the deepest ocean zone. There are about 8500 described living species under the phylum Porifera worldwide. Among them, over 98 % live in the ocean, and the rest inhabit in freshwater lakes, ponds, streams, etc. They perform an important role in filtering microorganisms and other contaminants from water.  They form a significant portion of many reef coral communities and provide brightly colored and come in a diverse array of shapes.

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