450 



LECTUIIE XIX. 



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

 the respiratory tract. 



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

 in great part, concentrated in the cephalothorax. The brain {Jig- \^Qoy 

 and 166, c) is a bilobed ganglion, sending forwards and upwards the 

 optic nerves (o) from its anterior angles, and below these, the two 

 large nerves (m) to the les 

 mandibles {m') : a short 

 and thick collar encloses 

 the narrow gullet, and 

 expands into a second 

 very considerable stel- 

 late or radiated gan- 

 glion (5), situated below 

 the stomach upon the 



plastron: it sends off Nervous system, Mygale. 



five principal nerves on each side ; the first {p) to the pediform 

 maxillary palpi ; the second (Z) 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 Mygale a third ganglion of 

 very small size is formed, from which the nerves diverge to supply 

 the teguments of the abdomen and its contents. The origin of the 

 mandibular nerves close to the optic ones from the superoesophageal 

 ganglion strongly indicates the antennal relations of the mandibles, 

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

 insects, their nerves from the subcesophageal 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. 



Many of the lower parasitic species of arachnids are blind : not 

 any of this class have compound eyes, but Galeodes and Pholcus 

 have their ocelli arranged in two lateral groups. The scorpions 

 have eight ocelli, two of which are situated near the middle line, 

 and three on each side near the anterior angles of the cephalothorax. 



In the spiders the ocelli are generally arranged in a group, upon an 

 eminence at the middle of the anterior part of the cephalothorax ; 

 they are generally eight, never less than six in number. The posi- 

 tion of the four median ones is the most constant; they generally 



