132 GENERAL PRINCIPLES OF ZOOLOGY. 



means of these branches extends into the various parts of 

 the body. We speak then of a ghstro-vascular system, be- 

 cause the alimentary canal itself takes on the function and 

 the branching arrangement generally characteristic of the 

 vessels or " vascula " (Fig. 60). 



Ccelom. The distribution of food may also be ef- 

 fected by the body-cavity or coclom, an extensive cavity 

 which, enclosing within itself most of the vegetal organs, 

 is lined by a special membrane, \\\<t peritoneum, or at least by 

 a true epithelium, and is bounded by the w r all of the digestive 

 tract and of the body. The body-cavity is probably only 

 an extension of the gastro-vascular system, for numerous 

 embryological researches upon many ccelomatous animals 

 have shown that the ccelom arises by evagination from the 

 digestive tract, and hence is only a part cut off from this, 

 an alimentary diverticulum, which has become indepen- 

 dent. 



Heart, Arteries, Veins, Capillaries. - -Finally, the 

 most complete method of food distribution is accomplished 

 by the blood-vessels, which, therefore, belong generally to 

 the groups of higher animals, and function in addition, 

 whether a body-cavity is present or not. Blood-vessels 

 are tubes with fluid contents, for transporting the oxy- 

 gen assimilated by the respiratory organs, as well as the 

 food absorbed from the digestive tract, and later giving 

 these up again to the tissues. Since such an interchange 

 of substances presupposes that the blood circulates in the 

 vessels, definite parts in the course of the blood-vessels are 

 contractile; they are covered by muscles which by their 

 contraction narrow the tube and push the fluid forwards. 

 In the lower forms wide areas in the course of the blood- 

 vessels are contractile; in higher animals a greater regu- 

 larity of circulation is reached; a definite specialized mus- 

 cular part of the course, the heart, alone moves the mass 

 of 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 enclosed in a special 



