ANIMAL ORGANIZATION. 103 



extensively employed as receptacles for retain- 

 ing them : forming, in the first place, sacs, or 

 pouches of various kinds for that purpose. The 

 ink-bag of the cuttle fish, the gall-bladder, and 

 even the stomach itself, are examples of this 

 kind of structure. The coats of these sacs, 

 being very extensible and elastic, readily ac- 

 commodate themselves to the variable bulk of 

 their contents. 



In the second place, we find membranes 

 composing tubes of various descriptions for con- 

 ducting fluids. Thus, in the higher classes of 

 animals, the whole of the body is traversed by 

 innumerable canals conveying different kinds 

 of fluids. These canals, when uniting into 

 trunks, or subdividing into branches, are called 

 Vessels (Fig. 26). 



27 



The fluids contained in vessels are never 

 stagnant, but are almost always carried forwards 

 in one constant direction. For preventing the 

 retrograde motions of the fluids passing along 

 these canals, recourse is had to the beautiful con- 

 trivance of valves. The iiuier membrane of the 



