OF THE CLASS AEACHNIDA. 457 



The organs of locomotion are all fixed to the cephalo- 

 thorax, and consist of eight pairs, strongly resembling those 

 of insects, and almost always terminated by two hooks ; their 

 length is in general considerable, and they easily break ; but, 

 as in the Crustacea, the stump, after having cicatrised, repro- 

 duces a new limb, which increases by little and little, and 

 ends by becoming similar to that of which the animal had 

 been deprived. The arachnida never present even the ves- 

 tiges of wings, and their abdomen is always completely de- 

 prived of locomotive appendages. 



553. It is in the anterior part of the cepha- 

 lothorax that we find the mouth and the eyes. 

 These latter organs are always simple, and in 

 considerable number ; we generally find eight of F . 496 

 them (Fig. 496), and we observe in each of them 

 a transparent cornea, behind which is a crystalline humour 

 or lens, and a vitreous humour, then a retina, formed by the 

 termination of an optic nerve, and an envelope of colouring 

 matter. We find nothing in respect of the instruments by 

 which the arachnida ascertain the presence of sounds, but we 

 have many proofs of the existence of this sense in these 

 animals, and it would even appear that some of them are 

 sensible to the charms of music. Touch is exercised chiefly 

 by the extremity of the limbs, and by the appendages with 

 which the mouth is provided. 



554. The nervous system of the arachnida presents 

 differences sufficiently remarkable; sometimes, as in the 

 scorpions for example, it is composed of a series of nine 

 ganglionary masses, reunited together by double cords of 

 communication, and forming a chain extending from one 

 extremity of the body to the other, and in an almost uniform 

 manner ; at other times, as in spiders, &c., we find all the 

 ganglions of the thorax united into a single mass (t, Figs. 

 497 and 500), whence proceed backwards two cords (c), which 

 proceed to terminate in a single abdominal ganglion (a, Fig, 

 500). Further, the general disposition of these parts is 

 always the same ; the anterior ganglions (c), situated in 

 front of and above the gullet, and considered most generally 

 as representing the brain of these animals, give origin to the 

 optic nerves anteriorly, and are continuous behind with the 

 cesophagal collar ; the other ganglions are situated under 

 the alimentary tube, and send nerves to the limbs, abdo- 

 men, &c. 



