CLASSES OF THE MOLLUSCA, 7 



2. Ill i\\Q gastei'oimda,'^ or snails, tlic under side of the body 

 forms a single musculai' foot, on which the animals creep or 

 glide. 



^^ 



Fig. 2. A G aster opod.\ 



3. The pterpodaX only inhabit the sea, and swim with a pair 

 of fins, extending outwards from the sides of the head. 



Fig. 3. A Fteropod. § 



The other mollusca ai-e acepJialoKS, or destitute of any dis- 

 tinct head ; they are all aquatic, and most of them are attached, 

 or have no means of moving from place to place. They are di- 

 vided into three classes, characterized by modifications in their 

 breathing-organ and shell. 



4. The bracJdapoda^ are bivalves, having one shell placed 

 on the back of the animal, and the other in front ; they have no 



* Gaster, the under side of the body. 



t Fig. 2. Helix desertorum. Forskal. From a Hving specimen in the 

 British Museum, March, 1850. 



X Pteron, a wing. 



§ Fig. 3. Hyaloea (ridentata. Lam., from Quoy and Gaimard. 



^ Brachion, an arm ; these organs were supposed to take the place of 

 the feet in the preceding classes. 



