11 INTRODUCTION. 



is nothing out a round and fleshy eminence, placed 

 on the anterior and upper part of the body, and ge- 

 nerally armed with two, or four, moveable horns, 

 or tentacula. These horns differ in their structure. 

 In the common snail, there are four, two large and 

 two small : they are hollow tubes, with eyes at the 

 end, and are capable of being extended, or drawn 

 in, at the pleasure of the animal. The Fossar of 

 Adanson, Helix ambigua, has but two, which are 

 solid, and immoveable, and the eyes are situated at 

 the anterior base of each. The position of the eyes 

 in the animal is, consequently, subject to variation ; 

 we find some that project, others that are sunk ; 

 some that are large, and others that are small. 



The mouth is very small in the animals of the 

 univalve shells ; it is generally a little groove, vary- 

 ing in its form and direction. In the snails, it is 

 furnished with minute substances, which may be 

 called teeth, and which are useful to them in feeding ; 

 and, in the animals of the genus Marex, it is a small 

 oval hole, from which issues a long retractile trunk, 

 terminated by a sucker, armed with short feelers. 

 It is with this trunk that they seize and kill such of 

 the smaller animals as are destined for their prey. 



The great muscle, or foot, which extends under 

 the neck, and part of the breast, of the animals of 

 univalve shells, is flat beneath, convex above, and 

 every moment changing its shape and direction. It 



