Almost all sponges have skeleton embedded in the Mesenchyme (loosely organised, mesodermal embryonic tissue which develops into connective and skeletal tissues, including blood and lymph).
Supports and protects the soft body parts.Serves as the basis of classification of Sponges., therefore, taxonomically also very significant.
Spicules:
Crystalline structures consisting of Spines or Rays that radiate from a point.
Secreted by Scleroblasts. Each ray is secreted by a group of two sclerocytes, one acting as a founder and the other as a thickener cell.
All kinds of spicules have a core of organic material around which calcium carbonate (Class: Calcarea) or colloidal silica (Class: Hexactinellida) is deposited.
Megascleres or Microscleres based on size.
Megascleres constitute the main skeleton.
Microscleres occur interstitially.
| Monaxon Growth along one axis. Straight needle like, rod like or curved. Ends may be pointed knobbed or hooked. Monactinal or Diactinal based on directions in which growth occurs. Occurs in both calcareous and siliceous types. | |
| Triaxon Three axes that cross one another at right angles to produce six rays. Hexactinal. Characteristic of only Siliceous sponges (Class: Hexactinellida). | |
| Tetraxon With 4 rays, each pointing in a different direction. Usually one of the three rays is elongated, giving the appearance of crown of 3 rays. When all rays are equal, the spicule is termed Calthrops. When one of the 4 is lost, it becomes triradiate (calcareous sponges). | |
| Polyaxon Spicules with several equal rays radiating from a central point. Common among microscleres. |
Spongin:
Spongin is organic, elastic substance resembling silk.
Scleroprotein containing sulphur and chemically related to collagen.
Insoluble, chemically inert, and resistant to protein digesting enzymes.
8-14% iodine.
May act as a cement connecting together siliceous spicules.
Keratosa: Spicules are completely absent, and only spicules are formed.
Spongin fibres are secreted by flask shaped mesenchyme cells called Spongioblasts.
Spongioblasts become arranged in rows and the spongin rod secreted by each fuses with those of neighbouring cells to form a long fibre.
Spongioblasts later become vacuolated and degenerate after having secreted a certain quantity of spongin.