70 



which open direct into the internal cavity of the tube. 

 The internal or gastral cavity is lined by the gastral layer 

 which is entirely composed of collar-cells or choanocytes. 

 The outer dermal layer is composed of flattened cells 

 (dermal epithelium) and amoeboid cells called amoebocytes 

 Besides these there are loose spicules. There are usually 

 some wandering amoabocytes in the middle jelly. 



What is the Structure of a Collar-cell or Choanocyte, and 

 what is its Function ? 



It is a rounded cell with a delicate projecting funnel 

 or collar which surrounds the lower portion of the long 

 flagellum. Both collar and flagellum can be withdrawn. 



By their action, the flagella draw the water in through 

 the inhalent pores ; and the food particles, caught in the 

 swirl inside the collar, are engulfed by the cell. The main 

 function of the choanocyte is to keep up the water currents ; 

 but by means of its collar it also secures the food which 

 is afterwards ingested and digested by the cell (intra- 

 cellular digestion). 



Indicate the Functions of the Amoebocytes. 



Some of them probably serve to transfer food and wastes 

 from layer to layer. Some become scleroblasts, spicule- 

 forming cells. Each triradiate spicule is made by three 

 scleroblasts. Other amoebocytes develop into genital 

 cells, which give rise to spermatozoa and ova. 



What are Porocytes ? 



Large specialised cells of the outer dermal layer which 

 pass through the wall and become hollowed out, and thus 

 form the inhalent openings or pores. 



