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CHAPTER IV. 



241 



Nutrition in the lower Orders of Jlnimals. 



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 nothing 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 

 ^■J^ 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 received and digested is laid open by a 

 longitudinal section, so as to show the 

 comparative thickness of the walls of this 

 cavity. The structure of these w^alls must be adapted not 

 only to prepare and pour out the fluids by which the food is 

 digested, but also to allow of the transudation through its 

 substance, probably by means of invisible pores, of llic nu- 

 tritious particles thus extracted from the food, for the pur- 

 pose of its being incorporated and identified with the ge- 

 latinous pulp, of which the body appears wholly to consist. 

 The thinness and transparency of the walls of this cavity 

 allow of our distinctly following these changes by the aid of 

 the microsco})e. Trembley watched them with unwearied 

 perseverance for days together, and has given the following 



