POLYPI. 



155 



is perfected by the addition of a closed capsule ; 

 and when it is first detected protruding from the 

 cell, it possesses all the parts of an adult polype, 

 and vibrates the cilia of its tentacula with as much 

 regularity and velocity as at any future period. 

 Before the polype is capable of protruding from the 

 aperture of the first cell, the upper part of that cell 

 has already extended outwards to form the rudi- 

 ment of a second: and so on, in succession, till the 

 whole structure is completed. 



The tentacula of polypi are exquisitely sensible, 

 and are frequently seen, either singly or conjointly, 

 bending their extremities towards the mouth, when 

 any minute floating body comes in contact with 

 them. When a polype is expanded, a constant 

 current of water is observed to take place, directed 

 towards the mouth. These currents are never pro- 

 duced by the motions of the tentacula themselves ; 

 but are invariably the effects of the rapid vibra- 

 tions of the cilia placed on the 

 tentacula. In the polypes of the 

 Flustra carbasea (Fig. 69), the 

 tentacula have each a single row 

 of cilia, extending along both the 

 lateral margins, from their base 

 to their termination.* Each po- 

 lype has usually twenty-two ten- 

 tacula ; and there are about fifty 

 cilia on each side of a tentaculum, 

 making 2200 cilia on each polype. As there are 

 above 1800 cells in each square inch of surface, 



* A portion of one of these tentacula is represented, highly mag- 

 nified, in Fig. 70. The lower figure (g) is the delineation of one of 



the gemmulcs of the same polypus, also greatly magnified. 



