252 MOLLUSCA. 



Tlieir 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 Pulraonea are distinguished from the other Mollusca by re- 

 spiring elastic air through a hole opening under the margin of the 

 mantle, and which they dilate and contract at will; they have no 

 branchiae, 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. 



Those wluch possess no apparent shell, form in the Linnaan system the 

 genus 



LiMAX, Lin. 

 Which is now divided into Limax, Jirion, Ldma, Vaginulus, Testacella, and 

 Parmacella. These animals are known by the common name of Slugs. 



In the terrestrial Pulmonea with complete and apparent shells, 

 the edges of the aperture in the adult are usually tumid. 



Helix, Lin. 



To this genus Linnteus referred all those species in which the aperture of 

 the shell, somewhat encroached upon by the projection of the penultimate 

 whorl, assumes a crescent-like figure. 



Helix pomatia, L. , common in the gardens and vineyards of France, with 

 a reddish shell marked with paler bands, an animal which in some places is 

 considered a delicious article of food. The genus is now variously subdi- 

 vided. The animals are what we term Snails. 



