POLYPES. 16-5 



the tube contracts, the fluid flows back into 

 the cavity, and the points of the tentacles con- 

 verge over the mouth. 



These parts are not only organs of sense, but 

 also serve many other purposes, particularly 

 those of prehension and motion ; and they very 

 probably assist in respiration, which appears 

 evidently connected with the alternate contrac- 

 tion and expansion of these animals. They are 

 also so constructed as to lay hold of every sub- 

 stance that floats within their reach, whether by 

 means of any gummy excretion like bird-lime, 

 as some suppose, or whether they are furnished 

 with very minute suckers by which they can 

 adhere to any substance, has not been ascer- 

 tained. Trembley observed, that when the 

 common polype of fresh water touched any little 

 animal with one of its long tentacular arms, it 

 was immediately arrested, and in spite of the 

 most violent efforts to liberate itself, which he 

 compares to those of a fish that had been 

 hooked, was held fast, and carried to the mouth 

 of the polype and swallowed. 



The body of polypes is formed of a kind of in- 

 spissated mucus, with confusedly agglomerated, 

 and probably nervous, molecules equally distri- 

 buted ; it is covered by no skin, is extremely con- 

 tractile, and forms an alimentary sac open at 

 one end, serving both for mouth and anal pas- 

 sage. The equal distribution of nervous mole- 



