40 SPONGES 



sea. The currents of water entering the incurrent pores are 

 laden with these minute organisms, which are taken up by 

 the collar cells of the radial canals and digested. There 

 are no special organs of digestion, but each of these cells 

 gathers and digests its own food. 



Method of breathing and excretion. — The same currents 

 of water that bring food' also carr;^ fresh oxygen, which is 

 giv6n up to the cells as the water passes over them. At the 

 same time, the cells give off carbonic acid gas, which is car- 

 ried outward by the water, and thus the sponge breathes. 



The particles of undigested food are carried out through 

 the osculum by the currents of water. ^^Tiatever waste sub- 

 stances the cells excrete are got rid of in the same way. 



Reproduction and life history. '■ — Grantia reproduces by 

 budding. That is, a bud forms on the external surface of 

 the body, which gradually increases in size and grows into a 

 new sponge individual. In the case of Grantia these bud 

 sponges break away from the parent, become attached, and 

 pass a solitary existence. In some sponges they remain 

 attached to the parents and thus there comes to be formed 

 a large colony of individuals. 



Grantia also reproduces in a sexual manner, although 

 "speciaUzed reproductive organs are not present. The 

 sexual elements will be found in the form of large spherical 

 bodies in the wall of the sponge. Fertilization takes place 

 here, and development begins, and the young embryos 

 escape into the sea water through the canals. For a while 

 the embryo is a free-swimming animal, but it finally fastens 

 itself to a rock and develops into the adult sponge." 



Structure of other sponges. — All sponges may be said 

 to have the three layers of tissue found in the simple sponges; 

 but, in other respects, the structure of the body in the 



