THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 59 



nerves, and, moreover, are not confined to the ganglion, but extend 

 for some distance into the interganglionic region of the nerve-cords 

 which connect together the ganglia of the ventral chain. 



Hardy's observations, therefore, combined with those of Milne- 

 Edwards, lead to the conclusion that in such a primitive arthropod 

 type as my theory postulates, each segment was supplied with 

 separate sensory and motor somatic nerves, and with a pair of nerves 

 of mixed function, devoted entirely to the innervation of the pair of 

 appendages ; that also, in the central nervous system, the motor 

 nerve-centres were arranged in accordance with a double set of seg- 

 mented muscles in two separate groups of nerve-cells. These nerve- 

 cells in the one case were aggregated into well-defined groups, which 

 formed the centres for the motor nerves of the markedly segmented 

 muscles of the appendages, and in the other case formed a system of 

 more diffused cells, less markedly aggregated into distinct groups, 

 which formed the centres for the imperfectly segmented somatic 

 muscles. 



Such an arrangement suggests that in the ancient arthropod type 

 a double segmentation existed, viz. a segmentation of the body, and 

 a segmentation due to the appendages. Undoubtedly, the segments 

 originally corresponded absolutely as in Branchipus, and every 

 appendage was attached to a well-defined separate body-segment. 

 In, however, such an ancient type as Limulus, though the segmen- 

 tation may be spoken of as twofold, yet the number of segments 

 in the prosoinatic and mesosomatic regions are much more clearly 

 marked out by the appendages than by the divisions of the soma ; 

 for, in the prosoinatic region such a fusion of somatic segments 

 to form the tergal prosoinatic carapace has taken place that the 

 segments of which it is composed are visible only in the young con- 

 dition, while in the mesosomatic region the separate somatic segments, 

 though fused to form the mesosomatic carapace, are still indicated 

 by the entapophysial indentations. 



Clearly, then, if the mesosomatic branchial appendages of forms 

 related to Limulus were reduced to the branchial portion of the 

 appendage, and that branchial portion became internal, just as is 

 known to be the case in the scorpion group, we should obtain an 

 animal in which the mesosomatic region would be characterized by 

 a segmentation predominantly branchial, which might be termed, as 

 in vertebrates, the oranchiomcric segmentation, but yet would show 



