518 COMPARATIVE ANATOMY chap. 



The occurrence of nidimentary abdominal limbs in the Arachnoidea proves that 

 the ancestors of these animals possessed extremities on the abdomen (at least on 6 

 abdominal segments). 



II. The Nervous System. 



The segmentation of the body is reflected in the segmentation of 

 the nervous system. The Scorpimiidic, which of all Arachnoidea show 

 the richest segmentation of the body, also possess the greatest number 

 of ganglia in the ventral chord, while on the other hand in the 

 Araneidm and Acaricke concentration both of body segmentation and 

 of nerve chord reaches its highest point. As in other Arthropoda 

 concentration of the nerve chord is due to displacements, fusings, 

 and to reductions of originally separate segmentally-repeated pairs of 

 ganglia ; these processes may be directly observed during onto- 

 genetic development. 



The brain is connected with the ventral chord by a short oeso- 

 phageal commissure. From the brain arise the optic nerves, and 

 also, in most eases, the nerves of the ehelieerse. The fact that the 

 chelicerse are innervated from the brain seems to oppose the assumption 

 that they are homologous with the mandibles of the Antennata, since the 

 latter always receive their nerves from the infra-oesophageal ganglion. 

 It has been found, however, that in the earlier stages of development 

 that portion of the brain from which the nerves of the chelicerag arise, 

 and which is often still distinctly separate in the adult animal, begins 

 to form in the embryo as the first post-oral pair of ganglia. These later 

 take part in the formation of the oesophageal commissures, or else even 

 fuse with the ganglionic rudiments of the segment of the frontal 

 lobes, i.e. with the rudiments of the actual brain. This process is evi- 

 dently similar to the fusing of the ganglia of the posterior antennae 

 with the brain in the Crustacea. In the Phalangidce, however, in op- 

 position to the other Arachnoidea, the nerves of the ehelieerse are said 

 to arise out of the anterior part of the great thoracic ganglionic mass. 

 A similar observation has recently been made in the A carina (Gama- 

 sidce), where "the mandible nerves arise out of two spherical gan- 

 glionic masses of the infra-cesophageal ganglion, and pass through the 

 supra-cesophageal ganglion." 



Throughout the Arachnoidea, even in the ventral chord of the 

 most richly segmented Scorpionidce and Solpugidce, all the ganglia of 

 the cephalo-thorax, and a number of the anterior abdominal ganglia, 

 fuse to form one great thoracic ganglionic mass, from which 

 arise the nerves for the second to the sixth pairs of extremities 

 and for the anterior abdominal segments. In the abdomen there 

 may be several separate ganglia {Scmpionidce, Fig. 363), or only one 

 or two (Thelyphonidce, Fig. 364, Solpugidm, Chernetidce, Phalangidce, 

 Mygalidce among the Araneidm, Fig. 374, p. 531). In the Dipneumones 

 (Araneidce) and the Acaridce (Fig. 365) on the contrary, the whole 

 central nervous system, the brain and ventral chord, form a single m.ass 



