THE EVIDENCE OF THE RESPIRATORY APPARATUS | 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 prosomatic and mesosomatic regions are much more clearly 
marked out by the appendages than by the divisions of the soma; 
for, in the prosomatic region such a fusion of somatic segments 
to form the tergal prosomatic 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 branchiomeric segmentation, but yet would show 
