36 ZOOLOGY SECT. 



supply of oxygen has to be got from the air, or, in the case of 

 aquatic animals, from the air dissolved in the surrounding water. 

 At the same time the carbonic acid has to be got rid of. In the 

 lowest animals as for instance Amoeba, and many of higher 

 organisation the oxygen passes inwards and the carbonic acid 

 outwards through the general surface of the body. But in the 

 great majority of animals there is a special set of organs the 

 organs of respiration having this particular function. In some 

 animals these organs of respiration are processes, simple or 

 branched, lined by a very delicate membrane, and richly supplied 

 with blood-vessels. Such processes are called gills or branchice ; 

 they are specially adapted for the absorption of oxygen dissolved 

 in water. 



In other animals the oxygen is obtained directly from the air ; 

 and in such air-breathing forms the organ of respiration is very 

 often a sac, either simple or compound, termed a lung. The 

 interior of this sac is lined with an epithelium of extreme delicacy, 

 immediately outside of which is a network of microscopic blood- 

 vessels or capillaries with thin walls ; and the oxygen readily passes 

 from the air in the cavity of the lung through its lining and 

 the thin wall of the blood-vessel into the blood. In other air- 

 breathing forms the organs of respiration are trachece, which are 

 ramifying tubes, by means of which the air is conveyed to all parts 

 of the body. In such forms, of which the Insects are examples, the 

 air is conveyed, by means of these tubes, from openings on 

 the surface of the body to all parts, and respiration goes on in all 

 the organs. 



In order that the air or water in contact with the surface of the 

 lungs or gills may be renewed, there are usually special mechanical 

 arrangements. In many gill-bearing animals the gills are attached 

 to the legs, and are thus moved about when the animal moves its 

 limbs. In others certain of the limbs are constantly moving in 

 such a way as to cause a current of water to flow over the gills. 

 In air-breathing forms there is usually a pumping apparatus, by 

 means of which the air is alternately drawn into and expelled 

 from the lungs. 



In a great number of animals there is in the blood a substance 

 called hcemoglobin, which has a strong affinity for oxygen ; and the 

 oxygen from the air, when it enters the blood, enters into a state 

 of loose chemical combination with it. In this state, or simply 

 dissolved in the fluid plasma of the blood, the oxygen is conveyed 

 throughout the body. 



Thus the blood, besides receiving the solid and liquid food from 

 the alimentary canal and carrying it throughout the body for 

 distribution, receives also the oxygen or gaseous food, and supplies 

 it to the parts requiring it. In all parts of the body in which 

 vital action is taking place chemical changes are constantly going 



