NEKVOUS SYSTEM OF AEACHNIDANS. 



379 



Fig. 190. 



in small currents into the sub -spinal vessel, thus intermingling the 

 venous and arterial blood, precisely as occurs in the abdomen. But the 

 circulation in the caudal prolongation of the heart yet remains to be 

 explained. We have already seen that the great dorsal artery in the 

 tail, above the colon, forms direct vascular communications around its 

 sides with the sub-spinal vessel upon the ventral surface, in which the 

 course of the blood is propelled forwards to the abdomen. It is certain, 

 therefore, that the action of the great chamber of the heart must impel 

 the blood at once in every direction, chiefly forwards, and laterally, but 

 also in part backwards through the caudal artery ; otherwise it would 

 be impossible for this structure to form its anastomoses with the sub- 

 spinal vein without occasioning two opposing currents in the same 

 vessel. 



(974.) In the nervous system of Spiders we observe that progressive 

 concentration of the nervous centres, which we have traced through 

 the lower forms of the HOMOGANGLIATA, carried to the utmost extent. 

 Spiders are appointed destroyers of insects, with which they maintain 

 cruel and unremitting warfare. That the 

 destroyer should be more powerful than 

 the victim is essential to its position ; that 

 it should excel its prey in cunning and 

 sagacity is likewise a necessary conse- 

 quence; and by following out the same 

 principles which have already been so 

 often insisted upon, concerning the in- 

 separable connexion that exists between 

 the perfection of an animal and the cen- 

 tralization of its nervous ganglia, we find 

 in the class before us an additional con- 

 firmation of this law. In Scorpions, in- 

 deed, the nervous masses composing the 

 ventral chain of ganglia are still widely 

 separated, especially those situated in the 

 segments of the tail : in the cephalo- 

 thorax they are of proportionately larger 

 dimensions, and moreover exhibit this re- 

 markable peculiarity, that, instead of being 

 united by two cords of communication, 

 there are three interganglionic nerves 

 connecting each division. It is in Spiders 

 that the concentration of the nervous 

 system reaches its climax ; for in them 

 we find the whole series of ganglia (ence- 

 phalic, thoracic, and abdominal) aggregated together, and fused, as it 

 were, into one great central brain, from whence nerves radiate to all 



Nervous system of the Spicier : 

 a a, encephalic ganglia, from which 

 are derived the optic nerves; 

 ee, thoracic ganglia, forming by 

 their coalescence the central 

 mass (c) ; n n, nerves supplying 

 the abdomen. 



