34 BRISTOL. ■ [Vol. XV. 



" The surprising thing is that we have left what seems to be 

 the exact equivalent of a trunk neuromere ; one pair of nerves 

 (i) and six ganglionic sacs, of which two are median and four 

 are lateral. Whether there is a pair of ' median nerve cells ' 

 connected with this part of the nervous system, I cannot say. 

 I have not found them, but my search has not been exhaustive. 

 The equivalence in other respects is so complete that there 

 seems to be no escape from the conclusion that tJie ganglionic 

 centers of the ventral cord are simple repetitions, element for 

 element, of the ' brain.' The nervous system is made up of seg- 

 ments of equal morphological value throughout. It must be 

 regarded then either as a series of ' brains ' or as a series of 

 ventral neuromeres, one or more of which have been carried 

 secondarily to the dorsal side, and which here take the place of 

 a brain that has been lost or confounded with the metameric 

 system. That a portion of neuromere II has suffered trans- 

 portation from the ventral to the dorsal side is certain ; but 

 the development of the supra-oesophageal system does not 

 permit us to believe that neuromere I was ever post-oral in 

 position. Allowing that it represents genetically the annelid 

 brain, as it certainly seems to do, the ventral cord must be 

 regarded as a chain of brains. The dorsal Y>os\tion of the brain 

 signifies nothing more than that the anterior end of the double 

 nerve cord has been bent upward from its prae-oral and ventral 

 position and slipped backward over the oesophagus." 



In the caudal region, although the concentration is quite as 

 great as in the head region, the elements of the neuromeres 

 are plainly resolvable. Each neuromere is complete in the 

 number of capsules and the nerve roots, which, however, are 

 here reduced to two. The whole nerve chain is divisible into 

 three portions : the head with six neuromeres, the trunk with 

 twenty-one, and the caudal disc with seven, making a total of 

 thirty-four neuromeres. Referring to PI. I (PI. IV here), the 

 innervation of a typical body somite is made clear. We find the 

 nerve divided into three distinct parts which we may designate 

 as anterior, middle, and posterior nerve, respectively. " A 

 glance will make clear one very interesting feature in the dis- 

 tribution of these nerves. They innervate three successive rings, 



