PORIFERA 



25 



in the open ; and yet I must apologise for still a little 

 further digression, for it is needed for the full understanding 

 of what is before us. We must have a little "animal 

 mechanics." 



Fig. 10 is a diagram showing the plan of a Sycandm. The 

 small arrows mark the direction of the currents of water. 

 The circular spaces in the wall of the sponge are the 



Fig. 10. Diagram, explain- 

 ing plan of a Sycandra 



Fig. 11. Diagram of a complex sponge 



chambers lined by flagellate cells, and where digestion 

 and absorption take place. Inside is a cavity, and at the 

 top, marked by a large arrow, is the osculum, by which 

 the water which has served the purpose of respiration, and 

 which has surrendered the food particles which it con- 

 tained, is forced out. 



The whiplike threads, flagellce, within the canals and 

 chambers, have a peculiar action : they wave to and fro, 

 but in the outward direction they move glidingly, as it 



