68 THE EVOLUTION OF MAX. 



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 mo uth 

 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-grains, 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 



PIGS. 182, 183. Ascula of a Sponge (Olynthus). Fig. 182, from the out- 

 side; Fig. 183, in longitudinal section: g, primitive intestine ; o, primitive- 

 mouth ; -i, intestinal layer } e, skin-layer. 



