CLASSES OF THE MOLLUSCA. 



2. Iii the gasteropoda* or snails, the under side of the body 

 forms a single muscular foot, on which the animals creep or 

 glide. 



Fig. 2. A Gastero^od.\ 



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

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



Fig. 3. A Pteropod. 



The other mollusca are acephalous, 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 brackiapoda^ are bivalves, having one shell placed 

 on the back of the animal, and the other in front ; they have nc 



* Gaster, the under side of the body. 



f Fig. 2. Helix desertorum. Forskal. From a living specimen in the 

 British Museum, March, 1850. 



\ Pteron, a wing. 



Fig. 3. Hyaloea tridentata, Lam., from Quoy and Gaimard. 



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

 the feet in the preceding classes. 



