NERVOUS SYSTEM AND HABITS OF ARACHNIDA. 211 



ample) it is composed of a series of eight ganglionic masses, 

 united together by double cords of communication, and forming a 

 chain extended from one end of the body to the other, in nearly a 

 uniform manner ; in other instances (as amongst the Spiders, &c.) 

 we find all the ganglia of the thorax united into a single mass 

 (, Figs. 437 and 439), whence two cords proceed backwards, 

 which go to a single abdominal ganglion. In other respects, 

 the general arrangement of these parts is always the same. 

 The anterior ganglia, situated before or above the oesophagus, 

 and most commonly considered as representing the brain of these 

 animals, give origin to the optic nerve in front, and are con- 

 tinuous behind with the collar of nerves surrounding the oesopha- 

 gus ; the other ganglia are situated below the alimentary tube, and 

 send nerves to the limbs, to the appendages of the abdomen, &c. 



743. The Arachnida are carnivorous, but in general confine 

 themselves to sucking the juices contained in the bodies of their 

 victims ; and in order to enable them more easily to effect the 

 capture of animals of whose strength they would be afraid, 

 Nature has endowed many of them with a poisonous apparatus. 

 The greater part are supported on Insects, which they seize whilst 

 alive ; some of them, however, live as parasites. Among the 

 first, the mouth is supplied with a pair of mandibles, furnished 

 with moveable hooks, or formed like a pair of pincers, with a 

 pair of thin or lamellated maxillae, each bearing a large feeler 

 more or less resembling a leg, and with a lower lip. Amongst 

 the parasitical Arachnida, the mouth has 



the form of a small trunk, from which 

 there issues a kind of lancet formed by 

 the maxillae. 



744. The moveable hook of the 

 mandibles has a small opening near its 



extremity, which is the orifice of the . 



excretory canal of the poison-gland al- labrum ; ma, maxm* ; P , 



, , ., ,. P, , . , maxillary palpi ; m, man- 



ready mentioned; and the liquid, Which dibles ; g, hook terminating 



it pours into the bottom of the wounds the mandibles - 

 made with the mandibles, almost immediately causes the destruc- 

 tion of the animal attacked ; but it is too weak to injure Man. 



p2 



