ARACHNIDA. 



255 



the great maxillary cheliform palpi, and to the four pairs of thoracic 

 legs: two slender continuations of the median columns are continued 

 along the jointed abdomen or tail, and seven small ganglions are de- 

 veloped upon them, from which and from the interganglionic chords 

 nervous filaments are distributed to the surrounding parts. 



The ventral continuation of the anterior aorta, which lies loosely 

 upon the dorsal aspect of the ganglionic chords, must be injected in 

 order that its branches, which accompany the nervous filaments, may 

 be distinguished from them. The vessel itself has been mistaken for 

 a nerve, and has been regarded by some as the motor, by others as the 

 res23iratory tract. 



In spiders the central masses of the nervous system are wholly, or 

 in great part, concentrated in the cephalothorax. The brain (^Jig. 109. 

 and 110. c) is a bilobed ganglion sending forwards and upwards the op- 

 tic nerves (o) from its anterior angles, and below these, the two lai'ge 

 nerves {m) to the mandibles : a short and thick collar encloses the 

 narrow gullet, and expands into a second very considerable stellate or 

 radiated ganglion (s), situated below the stomach upon the plastron : it 

 sends off five principal nerves on each side ; the first (/?) to the 

 pediform maxillary palpi ; the second (/) to the more pediform labial 

 palpi, which are usually longer than the rest of the legs and used by 

 many spiders rather as instruments of exploration than of locomotion ; 

 the three posterior nerves supply the remaining legs, which answer 

 to the thoracic legs of Hexapod insects. The nervous axis is 

 prolonged beyond this great ganglion, as two distinct chords, into the 

 beginning of the abdomen, where, in the Epeira diadema, it divides 



into a kind of cauda 

 equina; but in the My- 

 gale a third ganglion 

 of very small size is 

 formed, from which 

 the nerves diverge 

 to supply the tegu- 

 ments of the abdo- 

 men and its contents. 



Nervous system, Uygale. The Origin of the 



mandibular nerves close to the optic ones from the supra-oesophageal 

 ganglion strongly indicates the antenna! relations of the mandibles, 

 whilst the analogues of the maxillary and labial palpi receive, as in 

 insects, their nerves from the sub-cesophageal mass. The stomato- 

 gastric nerves are sent off from the posterior and lateral parts of the 

 brain and form on each side a reticulate ganglion, which distributes 

 filaments to the stomach. 



109 



