520 HISTORY OF THE HUMAN BODY 



tion is not in itself as fundamental a character as would appear 

 at first, and may be easily acquired by an animal group in 

 any one of several different ways. It is likely, for example, 

 that such a segmentation as that possessed by vertebrates may 

 have been gained through the muscular action of a previously 

 unsegmented form, and the fact that in vertebrate embryos 

 the segmentation first appears in the mesoderm, from which 

 the muscles are derived, furnishes a strong support for this 

 view. The oldest of the annelids, on the other hand, begins 

 life as an unsegmented larva, upon which the somites become 

 developed one after another through a sort of budding, a 

 process totally unlike that in which the vertebrate initiates its 

 segmentation. 



A second group of vermian forms from which the vertebrates 

 may have developed is that of the nemerteans, a group of 

 mainly marine worms, of uncertain affinities, but probably 

 allied to the platyhelminths (flat-worms). Here the nervous 

 system is not a ventral one, but consists of two lateral cords im- 

 bedded in the body wall, and often a smaller mid-dorsal cord, 

 the three being bound together by commissural nerves which 

 run around the animal (Fig. 141, A). A branching intesti- 

 nal nerve proceeds from one of these and is distributed to the 

 sides of the intestine ; and from the ventral portion of some of 

 the anterior commissural nerves small nerve branches appear, 

 also distributed to the intestinal wall. The anterior end Of 

 each lateral nerve is enlarged into a ganglion, from which a 

 few nerves proceed anteriorly. 



The manner in which such a nervous system may become 

 converted into that of a typical vertebrate may be readily seen 

 by a comparison of A and B of Fig. 141, the first of which 

 has already been referred to. Of the three longitudinal nerves 

 the dorsal one has become the central nervous system, and has 

 expanded its anterior end into a brain, while the two lateral 

 nerves have become subordinated to it, but persist in lower 

 vertebrates as the lateral nerves of the Vagus system, rami 

 laterales X, with which the long intestinal nerve is also asso- 

 ciated. The original ganglion of the lateral nerves breaks up 



