Skeleton:
The bodies of the majority of sponges are traveled by skeleton in the form of spicules, spongin fibers, or both. These skeletons are secreted by the cells of the body of sponges and so are called Autoskeleton.
Most of the sponges live in moving water and support their body with a well-formed skeleton structure. The skeleton is mainly mesohylar endoskeleton but sometimes an exoskeleton may develop over part or full of the body. The nature of the skeleton varies with the body structure of the species and their growth forms.
The mesohylar matrix is provided with calcareous or siliceous spicules or spongin or both. The spicules though principally located in the mesohyl but can project outside through the surface pinacoderm. Such projecting spicules commonly guard the oscula and sometimes the Ostia.
Spicules:
These are microscopic spines, crystalline in appearance having an axis of organic materials surrounding which are deposited either calcium carbonate or hydrated silica. Some sponges don’t possess spicules but they secrete organic spongin. Spongin and spicules occur together in most species of sponges.
Spicules are calcareous (CaCo3) or siliceous (SiO2) in origin. The spicules can broadly be classified into two forms –
- Megascleares
- Microscleres