II. Organs. 7. Vascular System. 8. Respiratory Organs. 27 



The minute structure of the vessels varies considei'ably ; the lining consists of 

 a single layer of flattened, cells, and these constitute the entire wall of the 

 capillaries; in other vessels the endothelial lining is usually suiTOunded by 

 connective tissue, smooth muscle fibres, etc., so that the walls of the large vessels 

 may be faWy thick. 



For the lymphatic vessels peculiar to the Vertebrata, see that group. 



8. Respiratory Organs. 



The cells of the metazoan body, like the Amceba, must have 

 oxygen in order to live : it must therefore be continually taken into 

 the body, and all the cells of the different organs must be supplied. 

 Further, the waste-prod,ucts, resulting from the constant combustion 

 going on in the cells, must be got rid of : one of these waste-products 

 is carbon-dioxide, a combination of carbon and oxygen (COj) 

 whose excretion alternates regularly with the absorption of oxygen, 

 whilst other waste matters are got rid of in other ways (see excretory 

 organs) . The taking in of oxygen, with the giving out of carbon- 

 dioxide, is known as respiration, and the organs performing this 

 function are called respiratory organs. Some animals (air- 

 breathing) obtain oxygen from atmospheric air, of which it forms 

 approximately one-fifth. Others, on the other hand, make use of the 

 free (dissolved) oxygen which occurs in all natural waters. 



The vascular system, when present, plays an important part in 

 connection with the introduction of oxygen and the removal of 

 carbon-dioxide. The blood in the vessels of the skin absorbs the 

 former, carries it through the body, giving it up on the way ; receives 

 the latter and returns with it to the skin, where it is expired and the 

 oxygen inspired again (cutaneous respiration) . In many animals, with 

 no respiratory organs, the digestive tract performs the respiratory 

 function. Air, or water containing air, is always swallowed with the 

 food, aad the oxygen is absorbed during its passage through the 

 digestive organs (intestinal respiration). Many of the lower animnls, 

 chiefly aquatic, but a few terrestrial [e.g., Earthworms), are destitute 

 of special breathing apparatus, and respiration is effected by 

 endosmosis, which goes on over the whole surface of the body. 

 These forms are almost always thin-skinned, i.e., they are without a 

 hard, thick cuticle, or any other covering difficult for oxygen to 

 penetrate, and are almost invariably of small size — a small body 

 has, of course, a relatively larger surface than a large body of the 

 same shape. 



Most animals are, however, provided with special organs of 

 respiration. The universally observed principle is that water con- 

 taining air, or atmospheric air itself, is brought into relation with a 

 large thin-skinned surface, through which oxygen is taken in in large 

 quantities, whilst carbon-dioxide is given off ; generally a capillary 

 net-work is distributed immediately beneath this skin. 



