50 LEISURE-TIME STUDIES. 



of these polypes. When the prey or food is swallowed, it 

 therefore passes into the interior (e) of the tubular body, 

 which evidently serves as a stomach-sac. Here the morsel 

 is digested or dissolved, and as the result of this process, a 

 fluid perfectly adapted for the nourishment of the polype is 

 formed. This fluid, or blood, is kept circulating through- 

 out the interior of the simple body by the constant move- 

 ments or vibrations of numerous minute processes named 

 cilia, which exist like a fringe on the lining membrane of the 

 body-cavity, and which therefore perform the functions of 

 the heart of higher animals. Thus it may be said that 

 every part of the hydra's body is brought directly into con- 

 tact with this nutritive fluid j since we note that the fluid is 

 transmitted from membrane to membrane and from cell to 

 cell by the process of imbibition. In this manner, then, does 

 the hydra repair the continual waste of its parts, this pro- 

 cess of waste being the inevitable result of the acts and func- 

 tions of every living being, and the invariable concomitant 

 of life itself. 



It is interesting to note that the green colour commonly 

 seen in the bodies of these animals is produced by the 

 development, within the cells of their tissues, of particles of 

 the substance named chlorophyll. This substance is that 

 which gives to plants their green hues ; and it forms a sub- 

 ject for remark, that the body of the hydra, a true and verit- 

 able animal, should be coloured with the same substance 

 which imparts the green hue to the duckweed to which it 

 attaches itself. The recognition of this fact may also serve 

 to show the close resemblance in chemical composition 

 which may exist between some animals and their plant- 

 neighbours. The exact use or function of these particles of 

 green colouring-matter found in the tissues of the hydra, 

 has not been determined. But it is not at all improbable 

 that the polype, through the possession of this green sub- 

 stance, may, like plants, be able to utilise certain of the 

 waste matters of its body, and notably carbonic acid gas, 

 and to elaborate such matters into useful nutriment. 



