68 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEA.L, 



acid (and water). The exchange of these two gases between the 

 blood and the external medium is the essential part of the respiratory 

 process, and is effected through organs which are suited for carrying 



on this process either in air or 



//V C 2-S > ~(^ V M i- n water. In the simplest cases 



the exchange of these two gases 

 takes place through the general 

 surface of the body ; and in all 

 cases, even when special respira- 

 tory organs are present, the outer 

 skin also takes part in respiration. 



FIG. 61. Diagram of the circulation in an 

 animal with a completely separated right 

 and left ventricle, and a double circulation 

 (after Huxley). Ad, right auricle receiv- 

 ing the superior and inferior vense cava3, 

 Vet, and Vci-, Dth, thoracic duct, the 

 main trunk of the lymphatic system ; Ad, 

 right auricle ; Vd, right ventricle ; Ap, 

 pulmonary artery ; P, lung ; Vp, pulmon- 

 ary vein ; As, left auricle ; Vs, left ven- 

 tricle ; Ao, aorta ; D, intestine ; L, liver ; 

 Vp', portal vein ; Lv, hepatic vein. 



FIG. 62. Diagram of the great 

 arteries of a mammal with, 

 reference to the five embry- 

 onic arterial arches (after 

 Eathke) . c, common carotids ; 

 c', external carotid ; c", inter- 

 nal carotid ; A, aorta. Ap, 

 pulmonary artery ; Aa, aortic 

 arch. 



Inner surfaces also may be con- 

 cerned in this exchange, especially 

 those of the digestive cavity 

 and intestine, or, as in the Echi- 

 noderms, in which a separate 

 vascular system is developed, the 

 surface of the whole body cavity. 

 Respiration in water obviously 



takes place under far more un- 

 favourable conditions for the introduction of oxygen than does the 

 direct respiration in air, because it is only the small quantity of 



