With few exceptions, the body of a sponge is supported by a skeleton made up either of spongin fibers, usually forming a network, as in the bath sponges (Spongia), or of spicules (little spikes) of silica or carbonate of lime.
Although we speak of the spicules as constituting a skeleton there are some remarkable differences between the skeleton of the majority of sponges and a skeleton as normally understood.
In the bath sponges and the others which have a fibrous skeleton, this is laid down in situ and is permanent. However, where the skeleton consists of spicules, as in the majority of the 2,500 known species of sponges, each spicule is laid down in the interior of the sponge and is then transported to its position by several amoebocytes.
The calcareous spicules are fashioned on a three rayed or triradiate plan, whereas siliceous or flinty spicules are of two kinds: those that are basically four-rayed and those that are six-rayed. These forms provide the primary divisions in the classification of sponges: the Calcarea; the Tetraxonida (four-rayed), and the Hexactinellida (six-rayed) with siliceous spicules; and the Keratosa with fibrous skeletons.
The greatest range of modification of the primary form of the spicules is seen in the Tetraxonida, but it is hardly less in the Hexactinel Lida.
In the Tetraxonida the siliceous spicules are modified to form forks, anchors, needles, stars, plates, keyholes, and a great variety of other forms. The important feature of these modifications is that it is possible to see in them an almost complete series indicating an evolution from the basic tetraxon (or hexaster, in Hexactinellida) to forms that, on their own, give no hint of the basic pattern.