240 MOLLUSCA. 



crawls, its shell is vertical, the columella crosswise on the hind part of 

 its luck , and its head passes under the edge of the opening opposite to 

 the columella. 



When the spire is salient, it inclines from the right side in almost 

 every species ; in a very few only does it project from the left when 

 they are in motion ; these are said to be reversed. 



It is observed that the head is always on the side opposite to that to 

 which the spire is directed. Thus it is usually on the left, and in the 

 reversed on the right. 



The organs of respiration, whfch are always situated in the last 

 whorl of the shell, receive the ambient element from under its edge, 

 sometimes because the mantle is entirely detached from the body along 

 this edge, and sometimes because it is perforated there. 



It sometimes happens that the margin of the mantle is prolonged in 

 a canal, in order to allow the animal to seek the ambient element with- 

 out protruding its head and foot from its shell. 



Most of the aquatic Gasteropoda, with a spiral shell, have an opercu- 

 lum, a part sometimes horny, sometimes calcareous, attached to the 

 posterior part of the foot, which closes the shell when its occupant is 

 withdrawn into it and folded up. 



Their organs of digestion vary as much as those of respiration. 



This class is so numerous that we have been compelled to divide it 

 into a certain number of orders, which we have founded upon the 

 position and form of the branchiae. 



ORDER I. 



PULMONEA. 



THE Pulmonea are distinguished from the other Mollusca by respiring 

 elastic air through a hole opening under the 

 margin of the mantle, and which they dilate and 

 contract at will; they have no branchise, but 

 a mere net-work of pulmonary vessels which 

 creep over the parietes of the respiratory cavity and chiefly on its ceiling. 



Some of them are terrestrial ; others are aquatic, but are compelled to visit 

 the surface from time to time for the purpose of opening the orifice of their 

 pectoral cavity, or to respire. 



The TERRESTRIAL PULMONEA have generally four tentacula; in two or 

 three only, of a very small size, the lower pair are not to be seen. 



