162 FOUNDATIONS OF BIOLOGY 



A. CIRCULATION IN THE LOWER VERTEBRATES 



In the higher plants the movement of water and food in 

 solution through the conducting systems is effected chiefly 

 by physical forces which are, to a certain extent, independent 

 of, though directed by, the activity of the plant cells. In 

 the higher animals, on the other hand, circulation is brought 

 about by an active system which forces as well as conducts 

 throughout the body what is to all intents and purposes a 

 fluid tissue. 



Many stages in the evolution of this elaborate circulatory 

 system can be traced from the lowest coelomate Inverte- 

 brates in which it consists merely of a single cavity or 

 several connected cavities filled with a fluid containing vari- 

 ous types of cells through forms in which more and more of 

 the spaces are replaced by definite tubes for the conduction 

 of the fluid. With the establishment of closed vessels, the 

 contractions of various organs and the movements of the 

 body as a whole can no longer be entirely depended on for 

 the movement of the fluid, and accordingly, in certain regions, 

 a muscular layer is developed in the walls of the tubes, which 

 by rhythmic pulsation forces the fluid along. Thus, for exam- 

 ple, in the Earthworm there is a fluid (coelomic fluid) within 

 the body cavity which is forced about by the movements of 

 the worm and bathes most of the internal organs; and also 

 a system of vessels, a part of which contracts rhythmically 

 and distributes the blood to the individual cells. (Figs. 66, 67.) 



In the higher forms a closed vascular system gradually takes 

 the ascendency and becomes what one ordinarily has in mind 

 when speaking of 'the circulatory system,' but the primitive 

 type of open system still functions as an auxiliary of no mean 

 importance even in Man. The highly developed Vertebrate 

 circulatory system, therefore, really consists of two parts. 



