CHAPTER VI 



THE CIRCULATION OF BODY FLUIDS 



Inasmuch as most activities of an organism are intermittent, 

 and the intake of material sources of energy localised, there 

 is usually in Metazoa some arrangement for keeping in motion 

 the body fluids and regulating this motion so as to meet with 

 the constantly changing requirements of the tissues. Thus 

 a consideration of the circulatory system may be conveniently 

 inserted in connexion with the sources of vital energy before 

 turning to the more specialised aspects of co-ordination in 

 the chapters which follow. 



The circulatory system subserves two functions : it dis- 

 tributes food to the tissues, and it supplies oxygen to them 

 removing carbon dioxide at the same time. The fact that 

 such intensely active organisms as dragon flies, being provided 

 with a respiratory apparatus which supplies oxygen directly 

 to the tissues, are able to exist with a circulatory system that 

 is practically vestigial, indicates that the primary importance 

 of the latter lies in meeting what Barcroft terms the call of the 

 tissues for oxygen rather than in distributing foodstuffs and 

 products of intermediate metaboHsm. It is also interesting 

 to note that the smaller representatives of groups the majority 

 of which possess a vascular system are those which are more 

 often found to be without a well -developed circulation. That 

 is to say, the necessity for a circulatory system seems to be 

 greater where the surface for intake of oxygen is relatively less 

 compared with the mass of the organism. The regulation 

 of the oxygen supply to the tissues by the blood is a subject 

 which has been investigated very little except in the higher 

 vertebrates. Most of the work on the circulatory system of 



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