'1 1 ZOOLOGY. 



Certain of the lower 

 animals are destined to 

 nourish themselves sole- 

 ly by liquids found in 

 plants or in the bodies 

 of other animals, on 

 which they live- as para- 

 sites. Many insects are 

 thus provided for; and 

 their mouth, instead of 

 presenting the ordinary 

 structures, is formed into 

 a lengthened tube or 



Fig. 9.-Bombyle Feint.* ' ^ , , f 



which they draw up the 



liquids they require (Fig. 9). The details of this curious struc- 

 ture will be explained when treating of the history of insects. 

 The liquid aliment quits the mouth, and descends imme- 

 diately into the stomach through the pharynx and gullet. 

 The solid part remains for a time in order to undergo the 

 action of mastication. 



Mastication. 



49. Mastication is performed by the teeth. 

 The Teeth. These organs are extremely hard substances, 

 resembling bone, firmly fixed into the alveolar edges of either 



C 



Fig. 10. Lower Jaw and Teeth of the Rabbit. 



jaw, and so as to act upon each other, or rather upon what- 

 ever is placed between them. In man, whom we select as the 

 example, each tooth is formed in a little membranous sac 

 lodged in the thickness of the jaw itself (Fig. 12). This sac, 

 which is named the dental capsule, is composed of two vas- 

 cular membranes, and encloses in its interior a small pulpy 



* The Variegated Bombyx. 



