STUDIES IN ANIMAL LIFE. 75 



lusk and Crustacean, to ■whicli the reader unfamiliar 

 witli tlie language of Natural History will have at- 

 tached but vague ideas ; and although I wanted to 

 explain these, and convey a distinct conception of 

 the general facts of classification, it would have 

 been too great an interruption. So I will here 

 make an opportunity, and fihish the chapter with 

 an indication of the five types, or plans of struct- 

 ure, under one of which every animal is classed. 

 Without being versed in science, you discern at 

 once whether the book before you is mathematical, 

 physical, chemical, botanical, or physiological. In 

 like manner, without being versed in Natural His- 

 tory, you ought to know whether the animal be- 

 fore you belongs to the Vertebrata, MoUusca, Artic- 

 ulata, Eadiata, or Protozoa. 



A glance at the contents of our glass vases will 

 yield us samples of each of these five divisions of 

 the animal kingdom. We begin with this Triton 

 (Fig. 17). It is a representative of the Vertebrate 

 division or sub-kingdom. You have merely to re- 

 member that it possesses a backbone and an inter- 

 nal skeleton, and you will at once recognize the 

 cardinal character which makes this Triton range 

 under the same general head as men, elephants, 

 whales, birds, reptiles, or fishes. All these, in spite 

 of their manifold differences, have this one charac- 

 ter in common — ^they are all backboned ; they have 

 all an internal skeleton; they are all formed ac- 

 cording to one general type. < In all vertebrate ani- 



