70 MANUAL OF ZOOLOGY sect. 



carbonic anhydride (carbonic acid gas) is constantly being 

 produced. The taking in of oxygen and giving off of car- 

 bonic anhydride is the process of respiration. The task of 

 facilitating the entry of oxygen and the passage outwards of 

 carbonic anhydride is in most of the Metazoa performed 

 by a set of organs known as organs of respiration ; but in 

 many respiration takes place through the general surface, 

 and special organs for carrying on this function are absent. 

 When organs of respiration are present, they are either 

 processes or gills {branchial) adapted for the respiration of 

 air dissolved in water ; or lungs or other cavities which are 

 adapted for the direct respiration of air. Through the thin 

 membrane lining the gill, or lungs, the oxygen passes and 

 enters the blood in vessels immediately underneath the 

 membrane, to be conveyed, like the food, throughout the 

 system and supplied to the several parts. At the same 

 time the carbonic anhydride, brought to the gill or lung by 

 the same means, passes outwards into the surrounding water 

 or air, and is thus got rid of. 



The blood consists of a fluid plasma, in which float 

 numerous cells — the blood corpuscles. Sometimes the blood 

 is colourless ; usually it is bright red, owing to the presence 

 of a red colouring matter, termed haemoglobin, which is 

 sometimes confined to certain of the corpuscles, sometimes 

 diffused throughout the plasma. Haemoglobin has a strong 

 affinity for oxygen, and is thus of importance in connection 

 with respiration. 



In order to carry on its functions as a conveyer of 

 nutriment and of oxygen throughout the body, the blood 

 flows in a system' of vessels — the blood vascular system — 

 which ramify throughout all the organs. Through this 

 system of vessels it is driven in a more or less regular 

 course, either by pulsating contractions of the muscular 



