68 THE EVOLUTION OF MAN. 



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

 differ 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-layer are coalescent and 

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

 sand-grams, etc., which 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 181, d). 

 When the Haliphysema 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 by the latter 



Fros. 182, 183. Vscnla of a Sponge (OlyntJw*) . Fijy. 182, from the out- 

 side; Pig 183. in longitudinal sfcHon : g, primitive intestine ; o, primitive 

 month ; i, intestinal layer ; e, skin-layer. 



