THE ANCESTRY OF THE VERTEBRATES 87 



In B, the mouth in. has shifted to a terminal position, the pros- 

 tomium is reduced and the brain has disappeared with its com- 

 missures. A new brain is forming ventrally by the concentration 

 of the ventral ganglia v.g. 1 to v.g. 7 , and the somites are becoming 

 crushed together in the head region. In C, the Vertebrate condi- 

 tion is attained by the mouth shifting on to the dorsal surface 

 (which becomes the new ventral surface) ; the brain is formed by 

 the further concentration of the ventral ganglia ; the somites are 

 still more crowded in the head and the dorsal parts of the posterior 

 somites fuse together to form the continuous perivisceral coelom. 

 Thus the so-called dorsum of the Vertebrate is shown in reality 

 to correspond to the ventrum of the Annelid and vice versa, if 

 this view is accepted. 



when, for instance, we imagined that we were ex- 

 hibiting a bold front to our enemies or turning our 

 back in reprobation, we were really acting in a 

 manner unworthy of our ancestry. A kind of moral 

 value has come to be associated with the ideas of 

 back and front, and we allow this moral value to 

 influence our morphological conceptions. But if we 

 put ourselves in the position of a worm which for 

 some reason is forced to adopt a more erect attitude, 

 or at any rate to forsake its creeping habit, we should 

 be hard put to it to know which surface to present to 

 a hostile and not incurious world. 



The complete loss of the original brain and pro- 

 stomium, which is involved in the Annelid theory 

 of Vertebrate descent, and its replacement by the 

 ventrally placed suboesophageal ganglion, is not such 

 a radical change as might be supposed. In the 

 Annelids there is a decided tendency for the seg- 

 ments behind the mouth, which are innervated by 



