OF THE EESPIEATION. 65 



arteries nor veins, and the nourishing fluid is spread about 

 in the interstices of the organs : still the circulation, such as 

 it is, is animated by the action of a vessel called dorsal, situ- 

 ated in the mesial plane of the body above the digestive tube 

 (Fig. 46). We shall consider, further on, the route followed 

 by the blood in the organism of those animals with a lacunar 

 circulatory apparatus. 



113. Worms. In the worms of the class annelides (such 

 as the leech and earth-worm) there exists, on the other hand, 

 a complete vascular apparatus ; but generally there is no 

 heart, properly so called, and the blood is set in motion by 

 movements of the vessels themselves, by contractions of the 

 principal vessels. Thus the course of the blood is much less 

 regular than in the various animals of which we have just 

 spoken, and frequently the direction is not constant. 



114. Zoophytes. There exists even in some zoophytes, 

 as in polyps, a kind of circulation, produced by the action of 

 the vibratile cilia with which the walls of the cavity, acting 

 at once as stomach and intestine, are provided. By means 

 of these cilia the contained liquids are kept constantly in mo- 

 tion. This cavity is sometimes single, but in some it sends 

 branches to various parts of the body. 



115. Such are the principal modifications hitherto ob- 

 served in the mode by which the circulation of the blood is 

 effected in various classes of animals. Let us now consider 

 the phenomena which happen whilst it passes through the 

 circulatory apparatus. 



OF THE EESPIBATION. 



116. The arterial blood, by its action on the living 

 tissues, loses its vital properties, which can only be restored 

 to it by being exposed to the action of the air whilst tra- 

 versing in vessels an organ adapted for this purpose. This 

 process of aeration is called respiration. The necessity for 

 this is proved by the simple experiment, if any such were 

 wanted, of placing an animal under the receiver of an air- 

 pump, and exhausting the air ; in a certain time the animal 

 dies asphyxiated. Wherever there is life, whether animal or 

 vegetable, air is essential. The term applies equally to aquatic 

 animals, which live by means of the air held in a kind of 

 solution or mixture by the waters in which they exist. 



117. The air we breathe, and which is essential to all 

 F 



