22 



MECHANICAL FUNCTIONS. 



tions are ascending on the one side, and descending on 

 the other. By these contrary motions the water would 

 be carried around the tentacula in a longitudinal direc- 

 tion, provided it was detached. But many of them be- 

 ing 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 produce a perpetual current of the fluid to the mouth 

 of the animal, and as the polype subsists on minute in- 

 sects, and particles of decomposing matter which it 

 takes from the water, this wonderful mechanism is evi- 

 dently designed to bring food to its mouth. 



The vibrations of these cilia, when the animal is vig- 

 orous, 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. 



To the Zoophytic order belong another tribe of ani- 

 mals called Hydra, which on some accounts are the most 

 singular and curious productions of nature. This ani- 

 mal 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, arte- 

 ries, or veins, or any other vessels whatever, all those 

 organs so essential to the existence of other animals be- 

 ing entirely wanting. 



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 1 What is composi- 

 tion of coral 7 In what manner are polypi multiplied 1 Explain fig. 7 

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

 towards their mouths 1 What purpose does this current answer to the 

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

 hydra? 



