PORIFERA. 167 



is motile and the fertilization takes place in the mesoderm. After 

 development has preceded a little way the young mass of cells 

 escapes and becomes free-swimming. It forms a gastrula (see 114 

 and Fig. 43), settles down and becomes attached by the end that 

 contains the opening (Fig. 43, 5), and gradually assumes the adult 

 condition (Fig. 42). The sponge may be looked upon, then, as a 

 modified gastrula, even in its adult condition. 



FIG. 44. Leucandra, a simple type of sponge. From Delage and Herouard. 



Questions on the Figure. Which is the attached and which the 

 free end of this sponge? Where is the osculum? How many 

 individuals are shown in the cut? 



1 86. The Conditions of Life in the Sponges. Sponges 

 are very much influenced by their surroundings. No 

 two are ever the same shape. They conform to the shape 

 of the object on which they are fastened. Individuals of 

 the same species differ greatly as they may happen to 

 grow in fresh and active currents or in sluggish water, 

 on rock or on muddy bottoms. The different species 

 differ all the way from forms which settle on rocks and 



