22 INTRODUCTION. 



III. The department of MOLLUSKS is divided into three 

 classes, namely : 



1. Those which have arms about the mouth, like the 



cuttle-fish, (Cephalopods,) fig. 47. 



2. Those which creep on a flattened disk or foot, like 



snails, (Gasteropods,) fig. 88. 



3. Those which have no distinct head, and are inclosed 



in a bivalve shell, like the clams, (Acephals.) 



The CEPHALOPODS mav be divided into 



/ 



a. The cuttle-fishes, properly so called, ( Teuthideans,) 



fig. 47. 



b. Those having' a shell, divided by sinuous partitions 



into numerous chambers, (Ammonites,) fig. 164. 



c. Those having a chambered shell with simple par- 



titions, (Nautilus.) 



The GASTEROPODS contain four orders : 



a. The land snails which breathe air, (Pulmonates.) 



b. The aquatic snails which breathe water, (Branch- 



ifers,) fig. 88. 



c. Those which have wing-like appendages about the 



head, for swimming, (Pteropods.) 



d. A still lower form allied to the Polyps by their gen- 



eral appearance, (Rhizopods or Foratninifera.) 



The class of ACEPHALS contains three orders : 



a. Those having shells of two valves, (bivalves,) like the 



clam and oyster, (Lamellibranchiates.) 



b. Those having two unequal valves, and furnished with 



peculiar arms, (Brachiopods.) 



