Sponges are regarded as most primitive metazoans with lower grade of body organizations. Body of sponges is made of cells, no tissues or organs found in their body. They are sedentary animals and look like plants. They can be differentiated from the protozoans in having cells and skeleton in the forms of spicules.
The described scheme of classification of the phylum Porifera follows scheme by Parker and Haswell (1964) and edited by Marshall and Williams (1974).
Definition of Porifera
A group of metazoan animal whose bodies are without any symmetry or radially symmetrical, without mouth and nervous system and whose bodies are provided with many pores, canals and chonaocyte cells, are recognized as Porifera or sponges.
Body skeleton, i.e., spicules and sponging fibres are the basis of the classification of Porifera.
- All sponges are aquatic, marine or freshwater; remain attached to some submerged substratum.
- Body flower-vase like or tubular, with radial symmetry or without any symmetry.
- Multicullular body is provided with many pores or Ostia.
- The space in between ectoderm and endoderm is filled with mesenchyme, i.e., the animal is diploblastic.
- Body is having canal system.
- Flagellated cells or choanocytes line the radial canal.
- Internal skeleton is made up of spongin fibre, siliceous or calcareous spicules.
- Reproduction sexual or asexual, asexual reproduction takes place by the formation of gemmules.
Scheme of classification
Phylum – Porifera
I. Calcarea (i) Calcaronea
II. Hexactinellida (i) Hexasterophora
III. Demospongia (i) Tetractinomorpha
Classification with reasons and examples
Class I- Calcarea
- All are marine, single in live in colony.
- Endoskeleton is composed of spicules made of calcium carbonate.
- Choanocyte cells are large.
- Osculum small surrounded by spicules.
Class Calcarea has been divided into two Sub Classes:
Sub Class (i) Calcaronea
- Nucleus of the choanocyte lies at the base of the collar.
- Flagellum arises from the basal granule attached with the nucleus.
- Spicules triaxon or 3-rayed, one ray is larger.
Examples- Sycon (= Scypha), Leucosolenia
Sub Class (ii) Calcinea
- Nucleus of the chonocyte lies at the base of the collar.
- Flagellum does not originate from the nucleus.
- Spicules tri-rayed, all rays are equal.
Examples- Clathrina, Petrobiona
Class II. Hexactinellida
- Flowre-vase like body.
- Spicules are composed of silica.
- Choanocytes are restricted to flagellated chamber.
Class II. Hexactinellida has been divided into two Sub classes:
Sub Class (i) Hexasterophora
- Spicules are smaller, triaxon and sixrayed.
- Spicules with pointed end.
Sub Class (ii) Amphidiscophora
- Spicules comparatively large.
- Both end of the spicule is provided with disc like plates.
Class III. Demospongia
- Endoskeleton may be formed by siliceous spicules or sponging fibres or both.
- Choanaocytes are restricted to spherical chambers.
- Spicules are having one to four rays.
Class Demospongia has been divided into two Sub Classes as:
Sub Class (i) Tetractinomorpha
- Spicules tetraxon types.
- No spongin fibre, only siliceous spicule present.
- Radially symmetrical body.
Examples: Cliona, Poterion
Sub Class (ii) Ceractinomorpha
- Endoskeleton made of spongin fibres only.
- Siliceous spicules when present monaxon type with one ray only.
Examples: Spongilla, Helichondra
Hartman and Goreau (1970) proposed addition of one more Class Sclerospongiae based on their researches on Jamaican sponges (Barnes- 1980). The characters of the Class are-
- Endoskeleton made of siliceous spicules and spongin fibres.
- Exoskeleton made of calcareous spicules.
- Star shaped canals system.
- Many Oscula.