XXli INTRODUCTION. 



II. Department of ARTICULATA. Animals whose body is 

 composed of rings or joints. It embraces three classes. 



1. Insects. 



2. Crustaceans, like the crab, lobster, &c. 



3. Worms. 



The class of INSECTS includes three orders. 



a. Those which have jaws for dividing their food (Man- 

 ducata), fig. 195. 



I. Those with a trunk for sucking fluids, like the but- 

 terfly (Suctoria), fig. 199. 



c. Those destitute of wings, like fleas (Aptera). 



The class of CRUSTACEANS may be divided as follows : 



a. Those furnished with a shield, like the crab and lob- 



ster (Malacostraca) . 



b. Such as are not thus protected (Entomostraca). 



c. An extinct race, intermediate between these two 



(Trilobites), fig. 378. 



The class of WORMS comprises three orders : 



a. Those which have thread-like gills about the head 



(Tubulibranchiata) . 



b. Those whose gills are placed along the sides (Dor- 



sibranchiatd) . 



c. Those which have no exterior gills, like the earth- 



worm (Abranchiata). 



III. The department of MOLLUSCA is divided into three 

 classes, namely : 



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



cuttle-fish (Cephalopoda). 



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



snails (Gasteropoda). 



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



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



The CEPHALOPODA may be divided into 



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



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



numerous chambers (Ammonites). 



c. Those having a chambered shell with simple partitions 



(Nautilus). 



