INTERNAL ANATOMY. 



97 



The ventral nerve-cord of the Annelids is no doubt in 

 part physiologically equivalent to the spinal cord of the 

 Vertebrates ; but since the two structures lie on opposed 

 sides of the body, it is difficult to regard them as morpho- 

 logically equivalent. Those who defend the Annelid-theory 

 have postulated the occurrence of a half-revolution of the 

 body in the supposed Annelid-like ancestors of the Verte- 

 brates, as a result of which they acquired the habit of per- 

 forming their locomotion, perhaps swimming, on their backs 

 so that the ventral surface was turned uppermost. In this 

 way, we are to suppose the original dorsal and ventral 

 surfaces became reversed. This phylogenetic acrobatic 

 feat with all its consequences is hard to imagine, and 

 there are other alternatives which make it an unnecessary 

 assumption. (See below, V.) 



The chief fundamental differences between the dorsal 

 spinal cord of Amphioxus and of Vertebrates generally, 

 and the ventral ganglionic chain of the Annelids, may be 

 summed up as follows : — 



As for the resemblances, in both cases nerves are given 

 off segmentally, and also giant-fibres are present, whose 

 function, however, is apparently very different in the two 

 cases. ^'^ 



