SPONGES 



39 



which these canals open outward, passes through fine open- 

 ings in the walls of the former directly into the radial canals, 

 from which it escapes into the cloaca and thence outward 

 through the osculum. 



Body walls. — The body walls of Grantia are much thicker 

 than those of the simple sponge, and the pores are 

 longer. There are three layers of tissue similar to those of 

 the simple sponge, already described. 



The inner layer, lining the radial canals, has its cells 

 peculiarly modified in structure, and they are known as 

 collar cells. Moreover, each collar cell possesses a long 

 vibratile flagellum. All of these flagella are constantly 

 waving with lashhke vibrations and thus maintain cur- 

 rents of water through the canals. 



Skeleton. — Like the simple sponge, Grantia has its body 

 supported by a mass of interlaced spicules, composed of 

 lime carbonate (Fig. 

 14). Two kinds of 

 spicules are always 

 to be found, — tri- 

 radiate and needle- 

 shaped. The tri- 

 radiate spicules are 

 interlaced and inter- ^• 

 woven in the middle 

 layer of the body 



walls so that they Fig. 14.- Sponge spicules. 



form a skeleton or supporting framework. The needle- 

 shaped spicules are found projecting through the outer 

 layer of cells all over the body. 



Method of feeding. — This sponge feeds upon micro- 

 scopic bits of plant and animal organisms, floating in the 



