68 THE EV^OLUTION 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 oj)posite 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-graius, etc., which serve to support tlie 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 Ifaliphysema 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 



Figs. 18'?, 183.— \scn1a of a Sponj?e {Olynthvs). F]>. 182, from the out- 

 side ; Fig. 183, in lono-itudinal spc^ion : g, primitive intestine j o, primitive 

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



