342 MR. H. M. BEENARD ON THE 



In the Spiders the six segments of the ccplialothorax have been evenly fused together ; hence the 

 ventral part of the ganglionic mass spreads back along the floor of the ceplialothorax, while the 

 supra-oesophageal portion lies quite anteriorly (PI. XXXI. fig. 3 o). The same ganglionic centres are 

 visible as are found in Galeodes (fig. 3 b), but in slightly different, and apparently more primitive, 

 positions. Owing to the forward projection of the cephalic lobes, in order to give the mandibles their 

 downward slope, the optic ganglia have been drawn to the front more over the cheliceral ganglia, which 

 appear to lie somewhat lower at the sides of the oesophagus than in Galeodes. Indeed, in young 

 spiders (Epeiridis ?) just hatched the cheliceral ganglion lies on each side well below the upper level 

 of the oesophagus, and seems, with the pedipalpar ganglion, to belong to the (Esophageal commissure. 



Again, the median (abdominal) ganglionic centres stretch backward beyond those of the last pair of 

 legs; indeed it seems in the Araneids as if it might be possible to count how many original segmental 

 ganglia the median abdominal ganglion represents, by counting the number of connective-tissue strands 

 which pierce it in the median line. 



In Scorpio the whole ventral surface of the cephalothorax has been pushed forward by the genital 

 aperture, with the result that the coxae of the pedipalps, except at their most distal ends, are no longer 

 in contact with the ground, the coxce of the 1st and 2ud pairs of legs being pushed forward under 

 them. This remarkable specialization of the cephalothorax has been reflected in the central ganglionic 

 mass, the ventral portion of which lies right forward directly under the supra-oesophageal portion 

 (fig. 3 c). This is well shown in Blanchard's ' Arachnides/ pi. vi. The forward thi-ust of the ventral 

 portion of the nerve-mass has brought the ganglionic centres of the nerves of the 1st pair of walking- 

 legs under those of the pedipalps, while those of the chelicerse are well above the level of the 

 oesophagus, and appear to be pushed round backward so as to lie in a plane posterior to that in which 

 lie the ganglia of the pedipalps and of the 1st pair of walking-legs. (Fig. 3 c is hardly pronounced 

 enough in this respect.) 



This thrusting forward of the ganglionic centres composing the ventral portion of the central 

 nerve-mass, owing to the shifting forward of the limbs, is still more marked iu the Phalangidae, in 

 which the nerve-mass lies very largely in the transverse plane (fig. 2>d). 



In the Pseudoscorpions the same ganglionic centres are discoverable. The enormous backward deve- 

 lopment of the coxae of the pedipalps, driving back the coxae of the remaining legs, finds some reflection 

 in the brain. The ganglionic centres of the six limbs all seem to radiate from the proximal end of the 

 root of the pedipalpar nerves (fig. 3 e) . The cheliceral nerves seem to rise from the upper ends of the 

 oesophageal commissures without being quite supra-oesophageal. The supra-oesophageal portion lies 

 forward over the anterior half of the ventral portion. 



The central nervous system of Gamasus, according to Winkler's description (76), is of great 

 morphological interest. While the concentration of the limbs at the anterior end of the body has 

 condensed the infra-cesophageal portion, the cheliceral ganglia are said to have retained their positions 

 at the lower ends of the ossophageal commissures. Inasmuch, however, as the muscles moving 

 the powerful chelicerre are inserted far back on the dorsal wall, the cheliceral nerve, in order to 

 reach them, has to traverse the supra-oesophageal portion. This, if correct, has been brought 

 about by the secondary shifting of the cheliceral muscles carrying the nerves, which originally pointed 

 forward, backward over the brain until they have become surrounded by brain-substance. This is a 

 very high specialization of a primitive condition, jjointing back to a time when the cheliceral ganglia 

 were infra-oesophageal. 



The nervous centres of the Pedipalpi at my disposal were not well enough preserved for the purpose 

 of making trustworthy comparisons. 



If, then, we start from the primitive arrangement in which all the segmental ganglia were ventral in 

 position, the nerve-centres of the Arachnids can be deduced from it by progressive forward concentration. 

 The first movement was doubtless made by the cheliceral ganglia, following the tilting forward of the 



