POLYPES. 173 



their food : so that they have, properly speaking, 

 neither a within nor without, both surfaces of their 

 alimentary canal being equally fitted for digestion. 

 This, however, is not so entirely anomalous as it 

 may at first sight appear ; for cuttings of some 

 vegetables, if planted inversely, will take root, the 

 top bearing the root, and the bottom the branches 

 and inflorescence. 



The fresh- water polype usually remains fixed 

 by its closed extremity to one spot, from which 

 it seldom moves, exhibiting no other trace of an 

 animated being than the motions of its arms ; 

 but when the want of light or heat causes it to 

 shift its quarters, it moves slowly by fixing 

 alternately, like a leech, its head and tail to what 

 it is moving upon. 



The majority of the marine polypes are at- 

 tached, in some way, to a calcareous support 

 formed by themselves, which is called by Amou- 

 reux, Lamarck, and other continental writers, 

 their Polypary , i1 and they are none of them 

 locomotive except the last order. 



4. The polypes of the second Order, the 

 sheathed polypes of Lamarck, 2 as the most im- 

 portant and interesting of this class of the animal 

 kingdom, I wish to leave last upon the reader's 

 memory. I shall, therefore, next make a few 

 brief observations upon those sponges and alcyons 



1 Fr. Polypier. * Polypi vaginati. 



