326 THE EVOLUTION OF MAH. 



exception of tlie intestinal epithelium. From the intestinal* 

 fibrous layer originates also the entire mesentery with all 

 the adjacent parts, the heart, the large blood-vessels of the 

 body, etc. (Plate V. Fig. 16). 



Let us now turn aside for a moment from this original 

 rudimentary intestine of Mammals, in order to institute a 

 comparison between it and the intestinal canal of those 

 lower Vertebrates and Worms, which we have learned to 

 recognize as the ancestors of Man. In the simplest Gliding- 



* ^^^ 



worm, or Turbellaria (Rhabdocoelum, Fig. 280), we find a 

 very simple intestinal form. As in the gastrula, the intes- 

 tine in these Worms is a simple pouch with a single open- 

 ing, which latter acts both as mouth and anus (wi). The 

 intestinal pouch has, however, differentiated into two sec- 

 tions, an anterior throat-intestine (sd) and a posterior 

 stomach-intestine (d). This differentiation becomes more 

 important in the Ascidia (Fig. 281) and in the Amphioxus 

 (Fig. 282), which connects the Worms with the Vertebrates. 

 In these two animal forms the intestine is essentially 

 identical; the anterior portion forms the respiratory giU- 

 intestine, the posterior forms the digestive stomach-intes- 

 tine. In both it develops, palingenetically, directly from the 

 primitive intestine of the gastrula (Plate XI. Figs. 4, 10). 

 But the original mouth-opening of the gastrula, or 

 the primitive mouth, afterwards closes, and in its place is 

 formed the later anus. In the same way, the mouth- 

 opening of the Amphioxus and of the Ascidian is a new 

 formation, as is the mouth-opening of Man, and generally, 

 of aU Skulled Animals (Graniota). The secondary forma- 

 tion of the mouth of the Lancelet is connected, as may be 

 conjectured with some probability, with the formation oi 



