326 ELEMENTS OF COMPARATIVE ZOOLOGY. 



tadpole and rat, and the spiral valve in the shark. In many 

 vertebrates the surface is still further increased by numerous 

 minute foldings and outpushings of the lining of the intes- 

 tine which, though so small as to be invisible to the naked 

 eye, still more than double the surface. 



With most food there are certain portions which are in- 

 digestible. These of course must be gotten rid of. In 

 the Coelenterates and flatworms the only opening through 

 which they can pass out is the same one by which they 

 entered, and so this opening, usually called the mouth, 

 serves at once as mouth and vent. In the higher forms 

 the alimentary canal becomes a complete tube with two dis- 

 tinct openings, one the mouth for the taking in of food, 

 the other anus or vent for the ejection of non-nutritious 

 portions. 



After its solution the food (nourishment) must be trans- 

 ferred to the parts which are to do the work. In the Pro- 

 tozoa the same parts which digest do the work. In the 

 sea-anemone and flatworms the pouching of the digestive 

 tract renders the transfer easy, for the pouches extend to 

 all parts. Above these forms we find circulatory organs 

 present, one of the functions of which is the carrying of the 

 dissolved food from the digestive tract to the working parts. 

 These circulatory organs are tubes through which the fluid 

 flows, but a flow can only be produced by some mechanism 

 which shall propel the fluid. In most cases this is effected 

 by muscles in the walls of the vessels, which by waves of 

 contraction force the fluid along. The tendency is con- 

 stantly towards a concentration of these pumping muscles 

 in one region, and thus a heart results. 



So far, we have traced the fuel to the working parts. 

 In order to do work the fuel must be oxidized, and this 

 means that oxygen must also be brought to these parts. 



