CHAPTER 15 



Circulation of Body Fluids 



T 



■ N THE PRECEDING CHAPTERS the phvsiological and biochemical adapta- 



■ tions to specific environmental factors have beer. :l'"'~cribed. We 

 M now pass to a series of functions which serve in reactions of the 



organism to the environment as a whole. We shall consider first the circulation 

 of body fluids, the mechanisms for the transport of food, oxygen, waste 

 products, hormones, phagocytic cells, and other materials from one part of 

 the animal to another. 



THE CIRCULATORY SYSTEM AS A WHOLE 



Morphological Types of Transport Mechanisms. A definitive and mutually 

 exclusive morphological classification of transport mechanisms is impossible, 

 because various animals have dissimilar structures serving the same function. 

 A working classification follows, with the recognition that it is incomplete 

 and that many degrees of intergradation exist. •^•■ 



Intracellular Transport. In protozoans there is usually much protoplasmic 

 movement, this streaming supplementing simple diffusion in the exchange 

 of materials between the organism and its environment. Considerable 

 churning and mixing is an accompaniment of amoeboid movement, and in 

 many ciliates food vacuoles follow a definite course through the organism. 

 In metazoans a certain amount of protoplasmic streaming occurs in most if 

 not all cells (see Ch. 17 for discussion of streaming). 



Extra-organismic Mechanisms of Transport. In many animals, particularly 

 sponges (Fig. 196, A) and coelenterates (Fig. 196, B), the water in which 

 the animals live provides an efi^ective medium for transport. Exchange 

 between body cells and external medium is as free as it is in the Protozoa. 

 The external medium passes in and through definite channels and may 

 transport food, oxygen, and possible coordinating substances from one region 

 of the animal to another, as in the coelenteron of coelenterates. In the 

 channels the fluid may be propelled by ciliary activity or by muscular move- 

 ment of the animal. Fluid from outside may enter special cavities and be 

 passed through these cavities in a definite fashion, to serve principally in 

 respiration and digestion. 



Transport in FluidTilled Body Spaces: Transport in primary body 

 CAVITIES. A special body fluid is transported, chiefly by muscular movement, 

 through various types of primary body cavities. A pseudocoelom is found 

 in such groups as nematodes (Fig. 196, C), ectoprocts, and rotifers. A 

 hemocoel comprises the principal passages in the "open" circulatory system 

 present in most arthropods and in many molluscs. Here a heart pumps 



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