210 



ORGANS OF SENSE IN ARACHNIDA. 



length is considerable and they easily break ; but, as amongst the 

 Crustacea, the stump, after being healed, produces a new limb ; 

 which grows by degrees, and becomes similar to the one of which 

 the animal had been deprived. The Arachnida never present 

 any vestige of wings : and their abdomen is always entirely 

 destitute of locomotive appendages. 



741. Upon the anterior portion of the cephalo-thorax, we 

 find the mouth and eyes. These last organs are always simple, 

 and of considerable number. We may generally 

 count eight, and in each of them may be distin- 

 guished a transparent cornea, behind which is found 

 a crystalline lens, and a vitreous humour ; then a 

 retina formed by the termination of an optic 

 nerve, and an envelope of colouring matter. 

 Nothing is known regarding the instruments by which the 

 sense of hearing operates in the Arachnida ; but there are 

 numerous proofs of the existence of such a faculty amongst 

 these animals ; and it would even appear that certain of them 

 are sensible to the charms of Music. The sense of touch is 

 exercised principally at the extremities of the legs, and by the 

 appendages with which the mouth is furnished. 



FIG. 436. 



FIG. 437. SECTION OF THE CEPHALO-THORAX OF A MYGALE, showing the arrangement of the 

 nervous sytem : ct, cephalothorax ; m, mandible ; g, moveable hook which terminates 

 it ; b, mouth ; a, eesophagus ; e, stomach ; ab, origin of abdomen ; c, cephalic ganglion ; 

 t, ganglionic mass of the thorax ; c<i, cords which unite it to the abdominal ganglia ; 

 no, optic nerve ; y, eyes. 



742. The nervous system of the Arachnida presents very 

 great differences; sometimes (amongst the Scorpions for ex- 



