CHAPTER VIII. 



RESPIRATION IN THE INVERTEBRATA. 



It is well known that the presence and absorption of oxygen 

 is essential to the life of every tissue, and that one of the 

 products of the action of oxygen on the tissues, &c., is the 

 production of carbonic anhydride, a gas which is inimical to 

 life. Even the lowest members of the animal kingdom re- 

 quire oxj^gen — without oxygen, no animal life. The Amceba 

 and Paramcecmm, when introduced into a medium contain- 

 ing no oxygen, or containing an excess of carbonic anhydride, 

 very soon die. In all animals there is an interchange be- 

 tween the gases of the organism and the gases of the medium 

 in which they live ; and this interchange, which is known as 

 respiration, is continuous throughout life. 



In the lowest forms no special mechanism is necessary for 

 facilitating the gaseous interchange ; for they absorb fluids 

 containing oxygen in solution. In higher forms, canals, 

 along which the air passes, seem to be necessary ; and in still 

 higher forms respiration is performed by the movement of 

 the branchifT, or by tracheae (air-tubes) and lungs. The 

 absoi'ption or respiration of oxygen is one of the first con- 

 ditions of nutrition. All organised beings absorb oxygen, 

 and this absorption goes on in all stages of the existence of 

 living matter. 



The organs (using the word in its widest sense) of respira- 

 tion differ considerably in different animals, but they have 

 all the same physiological function to perform — that of sup- 

 plying oxygen to the tissues and blood ; and the elimination 



