THE HIGHER ORGANISMS 135 



the respiratory system. The relatively simple organisms 

 absorb O from the fluids in which they live, at first by 

 surface absorption, then, when differentiated into an outer 

 derm and an inner gastric cavity, partly by absorption 

 from the external surface and partly through the gastric 

 contents, then by the transmission of the constantly 

 changing gastric contents through the gastro-vascular 

 system. When the blood becomes a permanently differ- 

 entiated fluid enclosed in vessels, some oxygenation is 

 effected through the surface of the body as the blood is 

 slowly moved about by the primitive heart, but as the 

 complexity of the organisms increases and large groups 

 of cells are set aside for various definite purposes, the 

 supply of oxygen thus secured becomes inadequate for the 

 support of the tissues, and it becomes necessary that special 

 oxygen-absorbing organs be provided and that the blood 

 be regularly brought to them. This necessitates an im- 

 provement in the blood itself, by which oxygen absorption 

 may be increased, and an improvement in the means of 

 circulating it in order that the freshly oxygenated blood 

 may not be free to mix with that whose oxygen has already 

 been exhausted that is, a separation of arterial and 

 venous blood. 



As has been shown, the pabulum supplied to the cells of 

 the most lowly forms of life differs from the surrounding 

 fluid in which the animal lives only in containing an 

 increased quantity of nutritious material available for 

 absorption or direct incorporation by the cells, this 

 condition persisting until the separation of the vascular 

 system from the digestive system is complete. The 

 nutrient pabulum then first deserves the name blood. 

 It continues for some time to be an aqueous fluid. 

 Occasionally one finds a few amoeboid cells from the 

 mesenchyme circulating in it and picking up any solid 

 particles that may accidentally enter. As the scale of 

 life is ascended the number of these increases and their 

 occurrence becomes more regular until in molluscs and 

 arthropods these amceboid "white corpuscles" are con- 

 stant elements of the blood. The blood, in the mean- 



