INTRODUCTION. 



In order to understand any science rightly, it needs that the student 

 should proceed to its contemplation in an orderly manner, arranging 

 in his mind the various portions of which it is composed, and endeav- 

 oring, as far as possible, to follow that classification which best accords 

 with nature. The result of any infringement of this rule is always a 

 confusion of ideas, which is sure to lead to misapprehension. So, in 

 the study of living beings, it is necessary to adhere to some determi- 

 nate order, or the mind becomes bewildered among the countless myriads 

 of living creatures that fill earth, air, and water. 



As a general arranges his army into its greater divisions, and each 

 division into regiments and companies, so does the naturalist separate 

 the host of living beings into greater and smaller groups. The present 

 state of zoological science gives five as the number of divisions of which 

 the animal kingdom is composed. These are called Vertebrates, Mol- 

 luscs, Articulates, Radiates, and Protozoa. Of each of these divisions 

 a slight description will be given, and each will be considered more at 

 length in its own place. 



1st. The Vertebrates include Man and all the Mammalia, the 

 Birds, the Reptiles, and the Fishes. 



The term "Vertebrate" is applied to them because they are fur- 

 nished with a succession of bones called " vertebrae," running along the 

 body and forming a support and protection to the nervous cord that 

 connects the body with the brain by means of numerous branches. 



2d. The Molluscs, or soft-bodied animals, include the Cuttle-fish, 

 the Snails, Slugs, Mussels, etc. Some of them possess shells, while 

 others are entirely destitute of such defence. Their nervous system is 

 arranged on a different plan from that of the Vertebrates. They have 

 no definite brain and no real spinal cord, but their nerves issue from 

 certain masses of nervous substance technically called ganglia. 



3d. The Articulates, or jointed animals, form an enormously large 

 division, comprising the Crustaceans, such as the Crabs and Lobsters, 

 the Insects, Spiders, Worms, and very many creatures so different from 



