VERTEBRATA. 



For the voluntary change of situation, most ani- 

 mals of this Division are furnished with limbs, usually 

 four in number, arranged in pairs. These vary in 

 form and function in the different classes. Food is 

 received and in some degree prepared for digestion by 

 the action of the jaws, the lower of which moves per- 

 pendicularly. In general they are armed with bony 

 projections called teeth, which cut or grind the food ; 

 but in the class of birds, and in the tortoises, these 

 are wanting, the jaws being encased in a horny beak. 

 The food passes from the mouth through a tube into 

 a sac called the stomach, where it undergoes certain 

 chemical changes, and receiving various secretions 

 from the body, goes into a lengthened membranous 

 tube, called the intestine, the sides of which are stud- 

 ded with innumerable little vessels, by which the nu- 

 tritious portion of the food is taken up, and conveyed 

 into the veins to form blood, the useless portion being 

 rejected. 



Before the supply of nutritious matter, which we 

 have just seen poured into the veins, can be made 

 available for the support of the body, it must undergo 

 some important changes. These are effected by the 

 admixture of oxygen, derived generally from the at- 

 mosphere ; but, in the class of fishes, from the water. 

 To understand in what manner this admixture takes 

 place, we must glance a moment at the circulatory 

 system. All the animals of this Division are copiously 

 supplied with a fluid essential to their existence, call- 

 ed blood, from which it is believed all other parts of 



