314 



ARACHNIDA. 



the heart drives a portion of the circulating fluid into the pulmo- 

 branchifE by the same effort which supplies the rest of the system, 

 and the blood so impelled to the respiratory organs becomes, after 

 being purified, again mixed up with the contents of the veins 

 which return it to the heart. 



(360.) In the nervous system of spiders we observe that pro- 

 gressive 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 essen- 

 tial to its position ; that it should excel its prey in cunning and 

 sagacity, is likewise a necessary consequence ; and by following 

 out the same principles, which have already been so often insisted 

 upon, concerning the inseparable connexion that exists between the 

 perfection of an animal and the centralization of its nervous gan- 

 glia, we find in the class before us an additional confirmation of 

 this law. In scorpions, indeed, the nervous masses composing 

 the ventral chain of ganglia are still widely separated, especially 

 those situated in the segments of the pig. 144. 



tail : in the cephalo- thorax they 

 are of proportionately larger dimen- 

 sions ; and, moreover, exhibit this 

 remarkable peculiarity, that, instead 

 of being united by two cords of 

 communication, there are three inter- 

 ganglionic nerves connecting each di- 

 vision. It is in spiders that the con- 

 centration of the nervous system 

 reaches its climax ; for in them we 

 find the whole series of ganglia, en- 

 cephalic, thoracic, and abdominal, ag- 

 gregated together, and fused, as 

 it were, into one great central brain, 

 from whence nerves radiate to all 

 parts of the body. The extent to 

 which centralization is here carried 

 will be at once appreciated by refer- 

 ence to the annexed figure (Jig- 144) : the encephalic masses 

 ja, a, whence the optic nerves distributed to the ocelli are de- 

 rived, are in close contact with the anterior part of a large 



