68 



THE EVOLUTION OF MAN. 



two primary germ-layers. These most simple Plant- Animals 

 dilTer from the gastrula principally in the fact that the 

 former are attached by one end (that opposite to the mouth- 

 opening) to the bottom of the sea, while the latter are 

 free. Moreover, the cells of the skin-la.yer are coalescent and 

 have included many foieign bodies, such as sponge-spicules, 

 sand-grains, etc., \vhicli serve to support the body-wall 

 (Fig. 180). The intestinal layer, on the other hand, con- 

 sists merely of a stratum of ciliated cells (Fig ISl, d). 

 When the Hcdiphysema is sexually mature, individual cells 

 of its entoderm assume the character of female egg-cells; 

 on the other hand, individual cells of its exoderm become 

 male seed-cells ; the fertilization of the former fiy the latter 



Figs. 182, 183.— Ai?eula of a Sponge (Ohjnflius). Eig. 182, from the ont- 

 sirle ; Fig. 183, in longitudinal section ; g, primitive intestine ; o, primitive 

 mouth ; (', intestinal layer ; e, skin -layer. 



