18 MECHANICAL FT/NOTIONS 



which the cilia stand, it will be observed that their mo- 

 tions are ascending on the one side, and descending on 

 the other. By these contrary motions, the water would 

 be carried around the tentacular in a longitudinal direc- 

 tion, provided it was detached. But many of them being 

 placed around the mouth of the polype, with the motions 

 of the cilia on the outside ascending, and those on the 

 inside descending, it is obvious that the effect is to pro- 

 duce a perpetual current of the fluid to the mouth of the 

 animal, and as the polype subsists on minute insects, and 

 particles of decomposing matter which it takes from the 

 water, this wonderful mechanism is evidently designed to 

 bring food to its mouth. 



31. The vibrations of these cilia, when the animal is 

 vigorous, are too rapid to be distinguished by the eye, 

 even when assisted by a microscope ; and it is only when 

 it becomes languid, and the motions diminished, that they 

 can be -seen. But the effect can be discovered by the 

 naked eye, by the motions of floating particles in the 

 water. 



HYDRA. 



32. To the zoophytic order belong another tribe of 

 animals called hydra, which, on some accounts, are the 

 most singular and curious productions of nature. This 

 animal consists of a stomach with tentacula for catching 

 its food, and nothing more. It exhibits not a trace of 

 either brain, nerves, or organs of sense of any kind ; nor 

 is there any parts corresponding to lungs, heart, arteries, 

 or veins, or any other vessels whatever ; all those organs 

 so essential to the existence of other animals, being en- 

 tirely wanting. 



33. Mr. Trembley, of Geneva, who watched the actions 

 of these animals with unwearied patience, for" days to- 

 gether, has given the following curious account of what he 

 discovered. 



What ornamental substance is made by polypi? What is composi- 

 tion of coral? In what manner are polypi multiplied? Explain Figs. 7 

 and 8. In what manner do polypi produce a constant current of water 

 toward their mouths? What purpose does this current answer to the 

 animal? What parts pertaining to other animals are wanting in the 

 hydra? 



