The body of sycon sponge  is made by outer derma! layer and inner gastral layer In between these two layers mesenchyme is present. This mesenchyme contains amoebocytes. The scleroblasts secrete spi­cules which from the skeleton of the sycon body.

These spicules are made by calcar­eous substance.

In sycon the spicules are two types.

1) Monaxon spicules

2) Tetraxon spicules

I) Monaxon spicules : These are three types,

a) Longer ones b) Shorter ones c) Club shaped.

These spicules are arranged in definite way in the body of sycon.

1. Long monaxons surround the osculum.

2. The short monaxons lie parallel to the radial canals.

3. The club shaped monaxons are grouped in the derma cortex. The club shaped monaxons are also called oxeates. They will usually project out of the body wall.

If the monaxon are pointed on one end and blunt on the other side they are called monactinal if they are pointed and both the ends they are diactinal ordiactines.

1. 'Style': It is monactinal monaxon spicule This is many types.

a) Tylostyle: One end is broad and the other end is pointed.

b) Acanthostyle: The spicule shows small spines on it.

2. Rhabd: It is diactinal monaxon spicule. This is many types.

a) Strongyles : The ends are round.

b) Tylote: The ends show ridges.

c) Oxea: The ends are pointed.

The analysis of these spicules revealed that they show Mg, Na sulphates in addition to CaCO3

II. Tetraxon spicules : These spicules will show 4 rays. These are present in the gastra! cortex. These tetraxons will loose one of their rays and look like triradiate.

Usually the tetraxons and triradiate spicules are present near spongocoel and one ray will project into the spongocoel.

Development of spicules:

In the mesenchyme scleroblasts are present . They give rise to spicules. Calcoblast (scleroblasts) will give calcareous spicule.

Development of Monaxon spicule :

The calcoblast is bi-nucleate. In between the 2 nuclei a central axis is formed, around this axis CaCO, is deposited. The cell absorbs CaCO3 from water.

The spicule will grow. Then the cell is divided into 2. The 2 cells will carry the 2 ends of the spicule in opposite directions. One cell is called founder'. It will maintain shape of the cell(spicule). Second cell is called thickner'. It will maintain thickness of the spicule. Thlis a 'Monaxon' spicule is formed. The 2 calcoblast cells will leave the spicule in Mesenchyme.

Development of Triradiate spicule : Rarely 3 caicoblast cells wiil come together and each one which  produce a 'Monaxon' spicule. The 3 spicules are united together and a triradiate' spicule is formed

The tetraradiate spicule develops like the triradiate spicule and the fourth ray is developed from the junction of the three.

Functions of the spicules in sycon sponge :

1) They will form a firm frame work of the body and give support to the soft body of the sycon.

2) Spicules give definite shape to the body.

3) They protect the body from disintegration by the wave action of the water.

4) They will keep open the dermal Ostia and canals throughout the life.