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BIOLOGY: GENERAL AND MEDICAL 



of the body. A new specialization makes its appearance, 

 however, for at least a part of the fluid thus distributed 

 does not return again to the stomach-intestine, but 

 leaves the body through pores guarded by special cells. 

 Here in intimate relation to the primitive circulatory 

 apparatus we find the inception of the essential function 

 of excretion or the elimination of effete matter. 



FIG. 47. Diagram to indicate the course of the blood in the nymph of a 

 dragon fly. Epitheca. a, Aorta; h, heart; the arrows show directions taken by 

 currents of blood. (After Kott>e.) 



The crudity of a system that permits the entire con- 

 tents of the alimentary canal to enter the circulating 

 pabulum is superseded by complete separation of the 

 digestive and circulatory systems; with corresponding 

 improvement in the quality of the systems thus sepa- 

 rated, the blood can no longer be propelled by move- 

 ments of the alimentary apparatus and for its proper 

 circulation it becomes essential that the great vessels 



