Sycon is a sedentary, aquatic or­ganism.The   sycon sponge structure is describe in previous topic. It shows diploblastic nature. It has 2 layers,

i) Dermal layer ii) Gastral layer, in between the 2 layers, mesenchyme is present. In the Mesenchyme many amoebocytes are present.



It is the outermost layer of the body. It is also called 'Pinacoderm'. It has 2 kinds of cells, i) Pinacocytes ii) Porocytes.

i) Pinacocytes : These are flat cells. They are polygonal in shape. They are

thin. They are nucleated. All the cells are closely fitted without inter-cellular spaces. This layer give protection to the body. They are highly contractile in nature. They are covering the spongocoel also.

ii) Porocytes : These are cylindrical cells. These cells contain a central pore. In the pinocoderm these cells are present, here and there. Wherever they are present pores are formed.

B) GASTRALLAYER : The inner layer of the body is called 'gastral layer'. It is also called 'choanoderm'. It contains 2 kinds of cells

(i) Choanocytes or collar cells (ii) Epithelial cells.

i) Choanocytes : These cells are called flagellated cells. These are round and nucleated. They have a long flagellum. At the base of the flagellum a protoplasmic collar is present. This collar will filter the water and catch the micro­organisms and the cell will digest them. Hence these cells are called nutritive cells. Because of the action of their flagella they draw water into the body and pushes it out of the body.


ii) Epithelial cells : in between the choanocytes thin epithelial ceils are present. Because of the epithelial cells the inner gastral layer is complete.

MESENCHYME : In between the pinacoderm and choanoderm mesenchyme

is present. In this mesenchyme many amoeba shaped amoebocytes are present. They perform different functions.

1) Myocytes : These amoebocytes are muscular in nature. They show contractions and expansions. They are present around the openings like ostia and osculum. When conditions are unfavourable, these amoebocytes will contract and close the openings.

2) Chromocytes : Amoebocytes contain different pigments and they are coloured. Because of these amoebocytes the sponge will get colour.

3) Thcsocytes : These amoebocytes will store food materials. During starvation period and asexual reproduction they are useful. They are round in shape.

4) Scleroblasts : These amoebocytes will produce spicules. They are three kinds.

1. Calco blast : They produce calcareous spicules.

2. Silico blast :They produce silieious spicules.

3. Spongio blast : They produce spongin fibres.

5) Archaeocyte : These amoebocytes are very important in sexual repro­duction. They give rise to sex cells. Male sex cell is 'sperm'. Female sex cell is 'ovum'.

6) Collencytes: These cells give slender branched pseudopodia. All these branches form a net work. Hence they are called connective tissue cells.

7) Trophocytes: These cells show pseudopodia. They collect food from collar cells and digest it. They distribute the digested food.

8) Gland cells: These cells produce an extension to the dermal layer and secret sticky matter which is useful to the sponge for attachment.