100 



GENERAL ERIXCIPLES OF ZOOLOGY 



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as in arthropods, llicse 'bodv ca\ilics' contain lilooil and arc in reality but ex- 

 pansions of tlic vascular svstcni. To such cavities the term IiiCiiiolwIc has been 

 gixen. 



Heart, Arteries, Veins, Capillaries. — Tlie most complete method 

 of food distribution is accomplished by the h/oihl-vcsnis, which, therefore, 

 belong generally to the higher animals, and 

 function, whether a body cavity be present or 

 not. Blood-vessels are tubes containing die 

 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 tlie 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 



^^. — P 



Fig. 04. Fti;. 05. 



Fig. 64. — Prndn'ca-lum laclnint (after lijima"). />, brain; (/, digestive tract with 

 c.Tcal branches; «, lateral iicr\c cords; />, pharynx \\iih sheath and mouth. 



Fig. 65. — Schema of circulation of the blood, a, arteries; c, ca|iillaries; /(, auricle; 

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



animals a greater regularity of circulation is reached; a definite s]iecial- 

 ized muscular part of the coarse, the heart, alone piropels the blood. 



The Higher Development of the Heart. — A free motioir of the heart 

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

 closed in a spiecial ca\ity (fig. 65). Hence the heart always lies either 



