414 ZOOLOGY. 



rings, as may be seen in scorpions (Fig. 413); sometimes of a 

 soft mass, globular, and without divisions, as, for example, in 

 spiders (Fig. 409).* 



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 tin 

 These latter organs are always simple, and in 

 considerable number ; we generally find eight of 

 Fig. 4,10. them (Fig 410), and we observe in each of them a 

 transparent cornea, behind which is a crystal- 

 line 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 instru- 

 ments 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 sen- 

 sible 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 dif- 

 ferences sufficiently remarkable; sometimes, as in the scor- 

 pions for example, it is composed of a series of nine gan- 

 glionary masses, reunited together by double cords of commu- 

 nication, 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. 411 and 111). 

 whence proceed backwards two cords (<?), which proceed to u-r- 

 ininatf in a single abdominal ganglion (a, Fig. 414). Fur- 

 ther, the general disposition of these parts is always the 



* The arachnides are divided into two orders, the pulmonaria and the 

 trai h.'iiria. Tin- first includes such genera as the aranea, the tarantula, 

 M'urpio. \e. Tin- second order includes the philangida, with its various 

 orders of siro troRulus, &.c., and the acarus, of which there are many sub- 

 genera and specie*.- U.K. 



