100 



GENERAL PRINCIPLES OF ZOOLOGY 



as in arthropods, these 'body cavities' contain blood and are in reality but ex- 

 pansions of the vascular system. To such cavities the term hcemoca'le has been 

 given. 



Heart, Arteries, Veins, Capillaries. The most complete method 

 of food distribution is accomplished by the blood-vessels, which, therefore, 



belong generally to the higher animals, and 

 function, whether a body cavity be present or 

 not. Blood-vessels are tubes containing the 

 blood, which transports the oxygen received 

 through the respiratory organs, as well as the 

 food absorbed from the digestive tract, and later 

 gives these up to the tissues. Since such an 

 interchange of substances presupposes that the 

 blood circulates in the vessels, definite parts of 

 the blood-vessels are contractile; they are covered 

 by muscles which by contraction narrow the 

 tube and push the fluid forwards. In the lower 

 forms wide areas are contractile; in higher 



FIG. 64. FIG. 65. 



FIG. 64. Dendroccelum lacteum (after lijima). b, brain; d, digestive tract with 

 CEecal branches; n, lateral nerve cords; p, pharynx with sheath and mouth. 



FIG. 65. Schema of circulation of the blood, a, arteries; c, capillaries; h, auricle; 

 k, ventricle; kl, valves; p, pericardium; v, veins. 



animals a greater regularity of circulation is reached; a definite special- 

 ized muscular part of the course, the heart, alone propels the blood. 



The Higher Development of the Heart. A free motion of the heart 

 is only possible when it is separated from the contiguous tissues and en- 

 closed in a special cavity (fig. 65). Hence the heart always lies either 



