INTRODUCTION. 21 



d. Those whose scales are entire, and whose fin rays 

 are soft, like the salmon, (Cycloids.) 



II. Department of ARTICULATES. 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 with a trunk for sucking fluids, like the butter- 



fly, (Suctoria,) fig. 62-64. 



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



ducata,) fig. 60. 



c. Those destitute of wings, like spiders, fleas, millipedes, 



&c., (Aptera.) 



The class 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. 156. 



The class of WORMS comprises three orders : 



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



( Tubulibranchiates.) 



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



sibranchiates.) 



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



(Abranchiates,) and also the Intestinal Worms. 



