66 THE VITAL FUNCTIONS. 



that calls for the continued exercise of their senses, 

 intelligence, and more active energies ; and for the 

 possession of those higher powers, which elevate 

 them so far above the level of the vegetable crea- 

 tion. 



Chapter IV. 



Nutrition in the lower Orders of Animals. 



The animals which belong to the order of Polypi 

 present us with the simplest of all possible forms 

 of nutritive organs. The Hydra, for instance, 

 which may be taken as the type of this formation, 

 consists of a mere stomach, provided with the sim- 

 plest instruments for catching food, — and nothing 

 more. A simple sac, or tube, adapted to receive 

 and digest food, is the only visible organ of its 

 body. It exhibits not a trace of either brain, 

 nerves, or organs of sens«, nor any part corres- 

 ponding to lungs, heart, or even 

 vessels of any sort ; all these 

 organs, so essential to the main- 

 '^'^^ ^^^K tenance of life in other animals, 

 being here dispensed with. In the 

 magnified view^ of the hydra, exhi- 

 bited in Fig. 241, the cavity into 

 w hich the food is received and di- 

 gested is laid open by a longi- 

 tudinal section, so as to show the comparative 

 thickness of the walls of this cavity. 



The thinness and transparency of the walls of 

 this cavity allow of our distinctly following the 



