BOOK III 

 THE CIRCULATION OF THE BODY FLUIDS 



CHAPTER XV 

 THE MECHANISM OF TRANSPORT 



THE unicellular organism floating in a nutritive watery fluid 

 lives by exchange between its body and the surrounding medium. 

 In the multicellular organism the deeper parts become too far 

 removed from the surface for a rapid exchange of material to take 

 place, and devices such as infolding are evolved, which lead the 

 medium into the inner recesses of the body. 



With the higher organization brought about by evolution, a limit 

 soon became set to such infolding, which interfered with the differenti- 

 ation of structure and division of labour necessary to render the 

 organism efficient in the struggle for existence. Hence, there came 

 about the development of a body cavity, or coelom, filled with an 

 internal medium, which, as blood or lymph, was at first made to 

 circulate by the general movements of the body. Later was evolved 

 a special pump the heart, or several hearts and a system of vascular 

 tubes, at first partly and then completely closed. By the aid of 

 these the internal medium could be driven with greater swiftness 

 and insure the better nourishment of every part. The internal tissue 

 of a Turbellarian worm, for example, is a loose aggregate of cells, differ- 

 entiated to a slight extent in structure and no doubt in function, 

 connected by strands of protoplasm. Between the cells are inter- 

 cellular clefts, which are connected with larger channels which 

 extend through the body, and act as circulatory channels. These 

 clefts and channels are filled with a fluid which carries the food and 

 oxygen supply to, and the waste products from the internal tissues, 

 and in every way acts as a simple blood. A to-and-fro movement 

 takes place as a result of the movements of the animal. The meso- 

 dermal cells in contact with the primitive channels become differ- 

 entiated in part into tissues, which form walls to these spaces. The 

 primary circulation spaces become specialized into continuous channels 

 which run the length of the body. The blood is driven to and fro 

 in this body cavity, or coelom, by the movements of the muscles of 

 the body, and, so propelled, bathes the respiratory tissues, the wall 

 of the gut, the nephridia and other structures. 



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