144 BIOLOGY: GENERAL AND MEDICAL 



organisms absorb their supply of oxygen from the 

 medium in which they live. This is, in fact, exactly what 

 the cells of the higher organisms do, except that the 

 medium in the first case is the water or atmosphere in 

 which the cells live and in the latter the blood that is 

 distributed to them. 



Among the lower forms of life nutrition and oxygena- 

 tion are intimately associated, and it is not until consider- 

 able complexity of structure is attained that it becomes 

 necessary to provide special organs for the purpose of 

 aerating the blood. 



Thus, in the porifera or sponges, the ciliated cells of 

 the entoderm, by causing currents of water to flow con- 

 stantly through the various body pores, keep the cells 

 of the animal constantly aerated. In hydra the cells of 

 the ectoderm probably absorb oxygen from the water 

 surrounding them, while those of the entoderm absorb 

 it from the water in the body cavity. In the higher 

 coelenterates with a primitive vascular system, the circu- 

 lating nutritious water distributed to the cells conveys 

 sufficient oxygen to support such cells as may not be 

 able to secure it from the surface of the body. 



Among the unsegmented worms where the water 

 vascular system is improved, respiration is still carried 

 on partly through the surface of the body and partly 

 by the primitive blood, but as the structural improve- 

 ment confines the blood in vessels, or at least completely 

 separates it from the contents of the digestive tube, some 

 adaptation must be provided for supplying oxygen to 

 the blood of the animal. 



In their most simple form these consist of slight bulg- 

 ings or projections of the surface corresponding with 

 thin points in the dermal covering of the animal, at which 

 the blood more easily takes up O and discharges its CO 2 

 than elsewhere. Such devices constitute the primi- 

 tive branchice or gills, the first of the special organs of 

 respiration. 



As the scale of anatomical complexity is ascended the 



