STRUCTURE 



59 



& it 



Figure 57. — Diagram. 

 To show parts of sponge. 



This is a physical process in which gases or liquids of 

 unequal densities, separated by a plant or animal mem- 

 brane, tend to mix and become alike, the liquids or gases 

 passing through the membrane. 

 Thus the food digested is 

 passed on and nourishes the 

 cells of the middle and outer 

 layers. The cells of the middle 

 region form spicules (spic'uls) 

 of lime (Figure 58) that pro- 

 ject through the other layers Figure 58. 

 and strengthen the whole body. 



The outer layer or ectoderm (ek'to-derm) serves as a 

 protective layer and with the help of the spicules gives 

 definite shape to the body. 



LABORATORY STUDY 



The sponge which we ordinarily handle is simply the skeleton, and is 

 easily kept from year to year. Examine several kinds of sponge skele- 

 tons and compare their shape, size, and the nature of the skeleton. How 



