POLYPI. 173 



minished vigour of the animal : their motions may 

 then be seen, ascending on one side of the ten- 

 taculum and descending on the other. (Fig. 70.) 

 All the cilia appear to commence and to cease 

 their motions at the same moment. The con- 

 stancy with which they continue would seem 

 to exclude the possibility of their being the 

 result of volition ; and they are, therefore, more 

 probably determined by some unknown phy- 

 sical cause, dependent, however, on the life of 

 the animal. But so retentive are they of the 

 power of motion, whatever may be its cause, 

 that if any one of the tentacula be cut off, its 

 cilia will continue to vibrate, and will propel 

 it forward in the fluid for a considerable time, 

 as if it had become itself an individual animal. 



A question arises with regard to the con- 

 stitution 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 one 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 indi- 

 vidual being characterised by having a single 

 rnouth, with its accompanying tentacula, and 

 yet the whole being animated by a common 

 principle of life and growth ? The greater num- 

 ber of naturalists have adopted this latter view, 



