110 



GENERAL PRINCIPLES OF ZOOLOGY. 



most of the vegetative organs. If the two halves of the coelom 

 approach, without uniting, dorsal and ventral to the gut, the 

 result is dorsal and ventral membranes, the mesenteries, which 

 support the alimentary canal. Of these the ventral is most fre- 

 quently, the dorsal least often, degenerate. In many invertebrates 

 the coelom plays an important role in nutrition since it contains a 

 lymphoid fluid, rich in proteids and containing cellular corpuscles. 

 It loses this significance the more the blood system is developed, 



FIG. 63. 



FIG. 64. 



FIG. (&.Leptoplana tremellaris. a, mouth ; b, buccal cavity ; c, opening of the head 

 of the pharynx into the buccal cavity ; d, central stomach ; e, branched ento- 

 dermal gut; /, ganglion; 0, testicle; ft, seminal vesicle; /c, uterus; /, receptaculum 

 seminis ; m, female sexual opening. 



FIG. 64. Schema of circulation of the blood, a, arteries ; c, capillaries ; 7i, auricle; 

 /c, ventricle ; fel, valves ; p, pericardium ; v, veins. 



and in the vertebrates, so far as nutrition is concerned, it is a 

 rudimentary organ. 



A sharp distinction should be drawn between the ccelom and other 

 cavities in the body. Not every ' body cavity ' is a coelom, but frequently 

 there occur large spaces which are entirely different in origin and in 

 relations. Frequently, as in arthropods, these 'body cavities' contain 

 blood and are in reality but expansions of the vascular system. To such 

 cavities the term hcemocode has been given. 



Heart, Arteries, Veins, Capillaries. The most complete 

 method of food distribution is accomplished by the Uood-vessels, 



