22 INTRODUCTION. 



III. The department of Mollusks is divided into tlirco 

 classes, namely : 



1. Those which have arms about the mouth, hke tlio 



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 may be divided into 



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

 fig. 47. 



h. Those having a shell, divided by sinuous partitions 

 into numerous chambers, [Ammonites^) fig. 161. 



c. Those having a chambered shell with simple par- 

 titions, (Nautilus.) 



The Gasteropods contain four orders : 



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

 h. 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 Foraminifera.) 



The class of Acephals contains three orders : 



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

 clam and oyster, (LameUibranchiates.) 



/•. Those having two unequal valves, and furnished with 

 peculiar arms, {Brachiopods.) 



