156 THE MECHANICAL FUNCTIONS. 



and the branches ol' an ordinary specimen present 

 about ten square inches of surface, we may esti- 

 mate tiiat an ordinary specimen of this zoopliyte 

 presents more than 18,000 polypes, 396,000 ten- 

 tacula, and 39,600,000 ciha. But other species 

 certainly contain more than ten times these num- 

 bers.! 



The currents determined by the vibrations of 

 these cilia ascend on one side of the tentaculum 

 and descend on the other. (Fig. 70.) If any one 

 of the tentacula be cut oft', its cilia will continue to 

 vibrate, and will propel the fragment forward in 

 the fluid for a considerable time, as if it had become 

 itself an individual animal. 



A question arises with regard to the constitution 

 of these Zoophytes, similar to that which has been 

 proposed with regard to trees, namely, what limits 

 should be assigned to their individuality ? Is the 

 whole mass, which appears to grow from the same 

 root, and which consists of multitudes of branches, 

 proceeding from a common stem, to be considered 

 as one individual animal, or is it an assemblage or 

 aggregation of smaller individuals : each individual 

 being characterised by having a single mouth, with 

 its accompanying tentacula, and yet the whole 

 being animated by a common principle of life and 

 growth ? The greater number of naturalists have 

 adopted this latter view, regarding each portion, so 

 provided with a distinct circle of tentacula, as a 

 separate animal, associated with its neighbours in 

 the construction of a common habitation, and con- 



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

 a single Flustra foliacea. Transactions of the Zoological Society 

 of London, vol. i. p. 11. 



