THE HIGHER ORGANISMS 133 



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 con- 

 tents, through the gastro-vascular system. When the 

 blood becomes a permanently differentiated fluid en- 

 closed 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 reg- 

 ularly brought to them. This necessitates an improve- 

 ment in the blood itself by which oxygen absorption may 

 be increased, and an improvement in the means of circu- 

 lating 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 inceases and their 

 occurrence becomes more regular until in molluscs and 

 arthropods these amoeboid ''white corpuscles" are con- 

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



