NUTRITION IN POLYPI. 75 



through its substance, probably by means of in- 

 visible pores, of the nutritious particles thus ex- 

 tracted from the food, for the purpose of its being 

 incorporated and identified with the gelatinous 

 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 microscope. Trembley 

 watched them with unwearied perseverance for 

 days together, and has given the following ac- 

 count of his observations. The hydra, though it 

 does not pursue the animals on which it feeds, 

 yet devours with avidity all kinds of living prey 

 that come within the reach of its tentacula, and 

 which it can overcome and introduce into its 

 mouth. The larvae of insects, naides, and other 

 aquatic worms, minute Crustacea, and even small 

 fishes, are indiscriminately laid hold of, if they 

 happen but to touch any part of the long fila- 

 ments which the animal spreads out, in different 

 directions, like a net, in search of food. The 

 struggles of the captive, which finds itself en- 

 tangled in the folds of these tentacula, are gene- 

 rally ineffectual, and the hydra, like the boa 

 constrictor, contrives, by enormously expanding 

 its mouth, slowly to draw into its cavity ani- 

 mals much larger than its own body. Worms 

 longer than itself are easily swallowed by being 

 previously doubled together by the tentacula. 

 Fig. 242 shows a hydra in the act of devouring 



