INTRODUCTION. 7 



are called arteries ; and the alternate dilatation and contrac- 

 tion of this important organ is the mechanism by which this 

 object is accomplished. In certain classes of animals, in which 

 the circulation is simple, the venous blood terminates in a 

 kind of reservoir or appendage to the heart named an auricle, 

 A muscular apparatus attached to this sinus propels the blood 

 which it receives through an orifice, into the cavity of the 

 heart. The ventricle, composed of thicker and stronger mus- 

 cular walls, is furnished with moveable valves, which prevent 

 the blood from returning into the auricle, while it is impelled 

 by the contraction of the ventricle into the artery. This ar- 

 rangement varies much, both in the mechanism and in the num- 

 ber of auricles and cavities in the ventricle, in different clas- 

 ses, and even in families of the same class of animals. 



The liquid prepared by the process of digestion requiring to 

 be submitted to the action of the atmosphere, or water contain- 

 ing air, to absorb the oxygen and deprive it of certain principles, 

 the function by which this is accomplished is called respiration. 

 The organ which performs this service is the lungs, through 

 which the blood is forced by the action of the heart. In ani- 

 mals doomed by their organization to live constantly in water, 

 respiration is effected by means of membranous laminae called 

 gills (branchice,) which separate the air from the water as it 

 passes over their multiplied surface. 



Among animals which appear to have no true circulation, 

 there exists another mode of respiration by tracheae or air-vessels, 

 by which the air is conveyed through the body in elastic canals ; 

 and in these animals it is through their integuments, which are 

 soft and easily permeable, or on their surface, that the respira- 

 tory function is exercised, as in vegetables. 



In many animals the mode of generation is not known. Of 

 others fixed to solid bodies, like vegetables by their roots, the 

 power of reproduction seems to be by buds or gemmce, or by 

 means of a separation, which operates naturally or accidentally, of 

 some parts of their bodies, in which are ultimately developed 

 the organs which at first were wanting. In all other animals 

 there are organs specially destined to generation. These organs 

 distinguish the males from the females In the greater part of 

 animals the sexes are distinct and separate in two different indi- 



