198 NERVOUS SYSTEM. 



It was, however, in the class arachnida that Treviranus 

 first pointed out, more than twenty years ago, in the ner- 

 vous system of the scorpion (Fig. 84. B,) the continuity of 

 this motor tract (84. B. , a,) over the upper surface of the 

 whole extent of the columns, and I have long shown the 

 same structure to pervade the articulated classes. The ner- 

 vous system, like most other parts of the arachnida, pre- 

 sents an intermediate condition of development betwixt that 

 of most insects and that of the higher Crustacea. While the 

 nervous columns of the scorpions, with their accompanying 

 series of ganglia, are lengthened in form, like those of lepi- 

 dopterous insects, the same parts in the spiders have the 

 concentrated form which they present in the highest crabs, 

 with their symmetrical ganglia concentrated in two points of 

 the body. The motor columns are large in the scorpion, as in 

 the scolopendra, and are here also easily separated from the 

 sensitive columns beneath them, excepting where they pass 

 over the surface of the ganglia, from which they cannot be 

 detached. The whole of the columns are less intimately 

 connected together, and the inter-ganglionic spaces are 

 larger in the cavity of the abdomen than in the round narrow 

 muscular tail, or caudal portion of the trunk. In this pos- 

 terior part of the body the motor columns are proportion- 

 ally more flat and more expanded over the ganglionic 

 chords, as they are likewise in Crustacea and other articulated 

 classes. Besides the cephalic, or supra-cesophageal ganglia 

 (Fig. 84. B. 1,) and the large infra-oesophageal mass of con- 

 centrated ganglia which radiates nerves to the five pairs of 

 legs, there are seven pairs of closely approximated ganglia 

 (84. B. 3 9,) of a lengthened form, disposed along the in- 

 ferior surface of the trunk. The motor or respiratory 

 nerves come off at the ganglia, as in the myriapods, and 

 not at a distance from the ganglia, as the inter-ganglionic 

 nerves of insects and Crustacea. Towards the caudal extre- 

 mity of the scorpion, the mixed nerves of the columns di- 

 verge suddenly in numerous minute fasciculi from the sides 

 of the ganglionic spaces (<84. B. g. 8.) But as we advance 

 in the trunk we find the whole of the mixed nerves coming 

 off in one large fasciculus from each side of each double 

 ganglion (84. B. 5, 4, 3.) The first and second pairs of ganglia 

 (84. B. 1, 2.) form a large, white, soft nervous mass occupy- 



