OF THE RESPIRATION. 



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 hy the action of 

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



Fig. 55. Circulation in Insects.* 



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

 of these cilia the contained liquids are kept constantly in 

 motion. 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 RESPIRATION. 



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

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



* The arrows point out the direction of the currents : a, dorsal vessel, in 

 which the blood moves from behind forwards ; b, principal lateral currents. 



