74 



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 

 simplest instruments for catching food, — and no- 

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

 receive and digest food, is the only visible organ 

 of the body. It exhibits not a trace of either 

 brain, nerves, or organs of sense, nor any part 

 corresponding to lungs, heart, or even vessels 

 of any sort ; all these organs, so essential to 

 the maintenance of life in other animals, being 

 here dispensed with. In the magnified view of 

 the hydra, exhibited in Fig. 241, 

 the cavity into which the food is 

 241 iP'^^f received and digested is laid open 

 by a longitudinal section, so as 

 to show the comparative thick- 

 ness of the walls of this cavity. 

 The structure of these walls must 

 be adapted not only to prepare 

 and pour out the fluids by which the food is di- 

 gested, but also to allow of the transudation 



