HISTORY
The initial advantage of becoming multicellular is the opportunity for increase in size. Larger organisms are less subject to attack; they have greater reserves within the body to withstand temporary unfavourable conditions. Multicellularity has been achieved independently perhaps four or five times in plants, Mesozoa, sponges and Metazoa.
Sponges belonging to phylum Porifera (L. Porus, pores+ferre, to bear) are sessile organisms with a low degree of individuality and cellular organisation. Sponges appear like plants being sessile and attached to rocks, shells and other solid objects. Most sponges are marine distributed from arctic to tropical seas, living from the low tide line to depths of 6 kilometers. One family (Spongillidae) is distributed in freshwater. Many sponges are drab coloured and others are brilliantly red, orange, yellow, violet, blue and black. The sponges have commercial importance being used in bathroom or in automobiles.
DIAGNOSTIC CHARACTERS OF PORIFERA
- Cellular grade of organization.
- Body bears several pores and hence the name Porifera.
- Presence of canal system.
- Skeleton of calcareous or siliceous spicules or horny spongin fibers.
- Commonly called as sponges.
- Asexual reproduction by gemmules and sexual reproduction by sperm and ova.
CLASSIFICATION OF PHYLUM PORIFERA
CLASS I CALCAREA
- Skeleton of calcareous spicules.
- Body asconoid, syconoid or leuconoid.
Order 1 Homocoela ( = Asconosa)
- Skeleton of calcareous spicules.
- Body asconoid, syconoid or leuconoid.
- Example :- Leueosolenia, Clathrina.
Order 2 Heterocoela (= Syconosa)
CLASS II HEXACTINELLIDA
- Six-rayed siliceous spicules.
- Exclusively marine.
Order 1 Hexasterophora
- Spicules hexasters.
- Usually attached to substratum.
- Example :- Euplectella, Staurocalyptus.
Order 2 Amphidiscophora
CLASS III DEMOSPONGIAE
- Spicules of Spongin fibers or Siliceous spicules or both.
- Large, solitary or colonial.
Order 1 Myxospongila
- Spicules absent.
- Simple structure.
- Example :- Oscarella, Halisarea.
Order 2 Tetraetinellida
- Tetraxon spicules.
- Spongin present.
- Example :- Chondrilla, Geodia, Chondrosia.
Order 3 Monaxonida
- Spicules monaxon.
- Fresh-water of marine.
- Example :-Suberites, Tethya, Cliona, Ephydatia, Chalina, Spongilla, Halichondria, Raispailia, Poterion, Haliclona, Microciona
Order 2 Tetraetinellida
- Spicules of spongin only.
- Horny sponges of massive sizes.
- Example :- Euspongia, Hippospongia, Hircinia.
