Discription: Internal or external structure which provides support to an animal body can be termed as skeleton system. This system is divided into two parts-
1. Endoskeleton- Inside the body
2. Exoskeleton- Outside the body
Spicular Endoskeleton of Sycon
This is made up of fine, spine like, microscopic connecting units which are termed as spicules. These spicules are basically made up of calcium carbonate and the animals in which these spicules are present known as calcaria. Arrangement of these spicules is related to their canal system which is a complex structure. On the basis of their shapes and size theae are divided into three-
A. Monoaxon- This type of skeleton is present in both calcareous and silicious sycons. These are long fine thin rod like structure having only one axix. These are again classified into large one rayed and small club shaped skeleton. Large encircled the apical aperture while small embded in dermal cortex.
B. Triaxon- This can be termed as triradiate spicule. Three rays are present in this. These are mainly present in glass sponges. These are situated on flagellated canals.
C. Tetraxon- Four rays are present in this. This is present in thick layer of gastric cortex which surrounds the cental spongocoel cavity.
Secretion of Spicules
These spicules develops from scleroblasts, which are present in mesenchyme. These are binucleated. In moniaxon only one spicule is involved. Formation of spicules begins as a deposition of a particle of calcium carbonate between two nuclei. Scleroblasts secreting calcareous and silicious spicules.
1. It conforms a particular shape to an animal.
2. It helps in preventing physical injuries.
3. Helps in movement of muscles.

Dr. Priyanka Dadupanthi