I STRUCTURE AND PHYSIOLOGY OF ANIMALS 35 



alimentary canal throughout the body, so that the various organs 

 may select from it the material which they require for the carrying 

 on of their functions. To carry out this office the blood is con- 

 tained in a complicated system of branching tubes or blood-vessels . 



The essence of the process of respiration, as we have alreadyseen, 

 is an interchange of oxygen and carbonic acid which takes place 

 between the tissues of an organism and the surrounding medium, 

 whether air or water. During the vital changes which go on in 

 the bodies of all animals, as in Amoeba, oxygen is constantly 

 being used up and carbonic acid being formed. The necessary 

 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 branchial ; 

 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 tracheal, 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 



d2 



