PRIMITIVE MULTICELLULAR ANIMALS 



end and an inner cavity known as the spongocoel. The walls of this hollow 

 cylinder are perforated by incurrent pores (Fig. 9.8); these are intracellular 

 canals passing through the cytoplasm of cells called porocytes. The external 

 surface, and the distal surface of the spongocoel just within the osculum, are 

 covered by a dermal epithelium of flattened cells. The remainder of the 

 spongocoel is lined by a single layer of flagellated cells called choanocytes 

 because of a protoplasmic collar surrounding the base of the flagellum on each 

 cell. Between the dermal cells and the choanocyte layer is a middle region 

 containing the spicules, with the cells that secrete them, the scleroblasts, and 

 connective-tissue cells (Fig. 9.9). Wandering cells called amoebocytes are 

 most numerous in this middle region, but they may occur in any part of the 

 body. These migrate by amoeboid movements like the white blood cells of a 

 vertebrate. The archaeocytes, a large type of amoebocyte, are also found 



Osculum 



Fig. 9.7. Olynthus stage of a calcareous sponge; 

 diagrammatic. (Redrawn from E. Haeckel, 

 1872, Die halkschwdmme .) 



Pores 



275 



