The soft body of sponge is supported by a firm network of endoskeleton. This support is made up of spicules of calcium carbonate, secreted by scleroblasts. The spicules are crystalline bodies with definite varied shapes. They are present in mesenchyme and remain embedded in gelatinous matrix. The spicules are enveloped by a sheath of organic material.
Sollas (1885) reported that it is in the form of calcite. It also contains magnesium, sulphate, sodium, water etc. Minchin has established that the scleroblasts are derived from dermal epithelial cells and later pass into mesoglea to secrete spicules. Each scleroblast can produce only one ray of the spicules. The tri- and tetra-radiate spicules are secreted by the corresponding number of scleroblasts. The spicules are of four types.
(i) Monaxon spicules: These are long needle-like spicules, large in number. They are one-rayed. They are arranged in circle around the osculum and form the oscular fringe.
The monaxon spicules can be further classified into two kinds, depending upon their size and shape.
(a) Long monaxon: They surround and guard the osculum.
(b) Short monaxon: They mostly lie parallel to radial canals.
(ii) Oxeote spicules: These are simple spear-like or club-like and project from the dermal cortex, over the polygonal elevation, opposite to the outer ends of radial canals, and give bristly appearance to the sponge.
(iii) Tri-radiate spicules: These spicules outnumber all other types. These are three rayed and are present along the flagellated canals. One ray of the spicule is pointed towards the closed distal end of the canal.
(iv) Tetra-axon spicules: These spicules have four rays. They occur alongwith tri-radiate spicules in the thick gastral cortex surrounding the spongocoel.
Fig. Spicules in Sponges