BODY FLUIDS AND CIRCULATION 85 



In higher groups tissue and body fluids of various kinds are invariably 

 present. These fluids are concerned with transporting oxygen, foodstuffs, 

 metabolic wastes, hormones, phagocytes, erythrocytes and other haemal 

 cells. In soft-bodied invertebrates the body fluids are also concerned with 

 maintenance of body turgor. Diffusion is such a slow process that in all 

 but the smallest animals some mechanism must exist for circulating body 

 fluids so that exchanges of substances may be facilitated. We shall now 

 consider the various kinds of body fluids, and the circulatory systems which 

 control their movements. Owing to the great diversity existing in the 

 organization of body spaces and circulatory systems in different animals, 

 any classification must be arbitrary and repetitive, but the following will 

 indicate the various categories which can be recognized. 



CLASSIFICATION OF BODY FLUIDS 



The External Milieu. This is utilized in some groups of marine animals 

 as a fluid medium subserving transport. Sponges are literally a network of 

 channels through which sea water is propelled by the unco-ordinated 

 activity of numerous flagella. The sea water carries oxygen and food to the 

 organism and waste products away. The hydraulics of this system are 

 described in Chapter 5 dealing with feeding and nutrition. In coelenterates 

 the gastrovascular cavity or coelenteron, with its ramifying passages, is 

 filled with sea water which forms an internal vehicle for transporting dis- 

 solved substances and foodstuffs to various parts of the body. In Cyanea, 

 for example, currents pass peripherally along the roof and return along 

 the floor of the gastrovascular channels (63). The water vascular system 

 in many echinoderms communicates with the exterior at the madreporite, 

 and acts as a hydraulic mechanism. In the cavities and channels of these 

 various animals the fluids are moved to and fro by ciliary activity or 

 muscular contractions. 



Tissue Fluids. These are important in all triploblastic animals as the 

 intimate milieu bathing the cells of the body and filling the spaces between 

 organs. It is through this medium that the cells receive or unload gaseous, 

 mineral and organic substances, and it is vis-a-vis the interstitial fluids that 

 ionic and other exchanges occur which are essential to the functioning of 

 nervous and contractile tissues (124). In simple metazoans, such as 

 platyhelminths which lack a body cavity, the fluids are confined to inter- 

 stitial spaces. The tissue spaces of those animals with open circulatory 

 systems, such as bivalve molluscs, are continuous with the haemocoele 

 and contain haemolymph. But in various higher forms with closed circula- 

 tory systems, especially the vertebrates, a distinct interstitial fluid perme- 

 ates the intercellular matrix, and communicates with the blood stream by 

 diffusion through the walls of the finest blood vessels, and by the lymphatic 

 circulation. 



There are few estimates of the volume of intercellular fluid in lower 

 animals. On the basis of thiocyanate determinations (injection of a known 

 amount of thiocyanate and subsequent estimation of its concentration in 



