INTRODUCTION. Ill 



often carries a calcareous, or cartilaginous operculum, 

 or lid, of a very variable shape, which seems to close 

 the opening of the shell, when the animal retires. 



The mantle is a muscular membrane, generally 

 thin, which lines the interior walls of the shell, and 

 envelopes the body, except the head and foot. In 

 some cases it lines not only the inside, but also the 

 outside of the shell, and in many it is prolonged in 

 a tubular form, which projects considerably before, 

 and assists the animal either in its movements, or in 

 fixing to solid bodies. The mantle has one or two 

 holes, differently situated, one for the absorption of 

 air, the other for the passage of the excrements. 



The Gasteropodes have a brain situated in the upper 

 part of the head, from which proceed two cords, 

 that unite and form a ganglion above the gullet. 

 It is from the brain and this ganglion, that all the 

 nerves proceed, which give sensation to the animal, 

 and life to all its parts. They have also a heart, 

 placed in the upper part of the body, and com- 

 posed of a single ventricle. Two vessels communi- 

 cate with it, one from the liver and other viscera, 

 and the other from what Cuvier calls the lungs. 

 The liver is very large, and divided into four lobes, 

 three anterior, and the fourth of a spiral shape, 

 which entirely fills the posterior part of the shell. 

 The organs of nutrition are the stomach and the 

 great and small intestine, all of them different parts 



