VAGAL AND HYPO-BRANCHIAL NERVES. 109 



we now see taking place in the arachnids is, I believe, beyond question. The 

 fact that the branchial appendages in arachnids, as nearly as we may deter mine, 

 belong to the same group of metameres as in vertebrates, and the fact that the 

 total number of branchial segments in Limulus and the merostomes is very 

 nearly the same as in vertebrates, i.e., seven appendages, four or five of which are 

 gill bearing, as against five or seven gill bearing arches for vertebrates is sug- 

 gestive, but perhaps of less significance than the fact that in both cases, there is a 

 much greater forward growth and concrescence of the structures on the haemal 

 surface than of those on the neural, thus producing that apparent lack of harmony 

 in the serial arrangement of nerves, neuromeres, gill arches, and myotomes, so 

 disturbing to the student of vertebrate cephalogenesis. 



The conditions become still more significant when we recall that they are the 

 inevitable results of very remote factors that are common to both types, such as 

 the absence of lateral plates to the mesodermic segments in the anterior part of 

 the head, the gradually increasing size of the yolk sphere, and the precocious 

 development of the forebrain. 



