17.2 



THE MECHANICAL FUNCTIONS. 



are invariably the effects of the rapid vibrations 

 of the cilia placed on the ten- 

 tacula. 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 ter- 

 mination.* Each polype has 

 usually twenty-two tentacula ; 

 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, and the branches of an ordinary 

 specimen present about ten square inches of sur- 

 face, we may estimate that an ordinary specimen 

 of this zoophyte presents more than 18,000 po- 

 lypes, 396,000 tentacula, and 39,600,000 cilia. 

 But other species certainly contain more than 

 ten times these numbers. f 



The vibrations of these cilia are far too rapid 

 to be followed 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~ 



* 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 gemmules of the same polypus, also greatly magnified. 



t Dr. Grant has calculated that there are about 400,000,000 

 cilia on a single Flustra foliacea. Transactions of the Zoolo- 

 gical Society of London, Vol. i. p. 11. 



