64 INTRODUCTION TO CONCHOLOGY. 



six tentacles, or feelers ; and the ejes, which never exceed 

 two in number, are situated either at the base of the tentacles, 

 or at the summit, or some intermediate part. These ten- 

 tacles are somewhat sensitive to the touch, and in the 

 Snails the mollusk withdraws them by inversion, for wliich 

 purpose a nerve reaches internally to the summit. 



Three ranges of cords represent the nervous system of the 

 Gastropods : the first pertaining to the head, the second to 

 the mantle, the third to the gills, each with its assigned 

 office, either as regards the well-being of the animals or the 

 place it is designed to occupy. 



The breathing apparatus consists, in those wliich inhabit 

 water, of two or more branchise, or giUs, either concealed or 

 exposed ; in such as dwell on land, or inliabit trees, it con- 

 sists of a net-bag or lung ; in those which are amphibious it 

 presents a modification of both. Some few of the maiine 

 species feed on sea- weed ; but more generally they are car- 

 nivorous, prepng ofttimes upon their brethren, or fulfiliing 

 the same office in the water, as jackals and hyenas, Nature^s 

 scavengers, perform on land. Most of the terrestrial kinds 

 are herbivorous, for wliich purpose they are furnished with a 

 horny armature on the upper lip only ; carnivorous moUusks, 

 on the contrary, are provided with a rasping plate or tongue. 



