460 EVOLUTION OF ANIMALS Part V 



face of the egg. The sperm enters the modified choanocyte, its own shape 

 changed by the loss of its tail and capsule-like cover. It passes through the 

 choanocyte, enters the egg and fusion of the male and female nuclei finally 

 occurs. 



After fertilization, the egg divides completely and at the 16-celled stage the 

 embryo is a disk-shaped cushion of cells (Fig. 22.5). The eight cells next to 

 the collared cells are the layout of the future outer cover or epidermis of the 

 sponge. The other eight cells are the future collared cells. The latter divide 

 rapidly and develop flagella. In this stage the embryo of calcareous sponges, 

 now a hollow sphere, makes its way into the water currents in the parent 

 sponge and is borne out of the excurrent opening as a free-swimming animal. 

 Later, the layer of collared cells bends inward and the epidermal layer grows 

 over it forming an outer sac around it. By this stage the young sponge has 

 attached itself to a rock or seaweed and settled down for its further develop- 

 ment and to a life of complete dependence upon the currents of the sea. 



Sponges reproduce asexually by budding and branching somewhat after the 

 fashion of plants. This habit produces the familiar "fingers" of sponges, as in 



B 





Fig. 22.6. Fresh-water sponge. A, living Spongilla, spread over a stone. The out- 

 lines of the water canals are faintly visible. Spongilla often covers submerged twigs 

 and if exposed to sunlight is green with algae that grow within its cells. Bl , diagram 

 of a section of the wall of a fresh-water spicule-bearing sponge. B2, microscopic 

 spicules within the cells which formed them. Greatly enlarged. (Courtesy, Mor- 

 gan: Fieldbook of Ponds and Streams. New York, G. P. Putnam's Sons, 1930.) 



