POLYPI. 



129 



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

 already extended outwards to form the rudiment 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 altogether, bending their 

 extremities towards the mouth, when any minute floating 

 body comes in contact with them. When a polype is ex- 

 panded, a constant current of water is observed to take 

 place, directed towards the mouth. These currents are 

 never produced by the motions of the tentacula themselves; 

 but are invariably the effects of the rapid vibrations 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 

 polype has usually twenty-two ten- 

 tacula; and there are about fifty cilia 

 on each side of a tentaculum, makins: 

 2200 cilia on each polype. As there 

 are above 1800 cells in each square 

 inch of surface, and the branches of an ordinary specimen 

 present about ten square inches of surface, we may estimate 

 that an ordinary specimen of this zoophyte presents more 

 than 18,000 polypes, 396,000 tentacula, and 39,600,000 ci- 

 lia. But other species certainly contain more than ten times 

 these numbers.t 



The vibrations of these cilia are far too rapid to be fol- 

 lowed by the quickest eye, even when assisted by the most 

 powerful microscope, and can be detected only at the times 

 when they have become comparatively languid, by the di- 

 minished vigour of the animal: their motions may then be 



• 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. 



f Dr. Grant has calculated that there are about 400,000,000 cilia on a sin- 

 gle Flustra foliacea. Transactions of the Zoological Society of London, 

 Vol. i. p. 11. 



Vol. I. 17 



