Ill 



SYSTEMATIC ZOOLOGY IOR TEACHERS. 



are fertilized, and proceeds as described on page 177, and illustrated by fig. 

 HO. 



After t^e einbyro lias advanced as far as the last stage given, another 

 change takes place. The egg which has hitherto been spherical in form, 

 becomes slightly flattened at one point. This flattening proceeds and finally 

 develops into a cavity which, becoming deeper and deeper, at last occupies 

 the greater portion of the body of the egg, which has now become somewhat 

 elongated. h>y this arrangement it will be seen that the cells are crowded 

 out of the center of the body of the young sponge, leaving a cavity. (See 

 fig. 107. ) This cavity serves as kind of stomach into which food is drawn 

 by the action of cilia that are developed from the outer walls and along the 

 margin of the cavity. Thus it can be seen that the young sponge begins 



Fig. 106. 



Creeping Sponge. 



life as a kind of anamated stcmach ; this stage of its existence is known 

 as the gastiula stage, and all of the lower foims of animal life up to ver- 

 tebrates acquire nutrition in the earlier stages of their lives in a similar way. 

 The young sponge moves about for a time, by the aid of cilia, which are 

 developed on the outer walls then settles down in seme situation best adapted 

 for its growth and development. 



Sfokge Flesh. 



The embryo sponge, when it settles down in what will be its peimanent 

 resting place, is composed, as we have seen, of cells of protoplasm. These 

 cells go on accumulating, but from this point on gradually change ; become 



