130 THE MECHANICAL FUNCTIONS. 



seen, ascending on one side of the tentaculum and descend- 

 ing on the other. (Fig. 70.) All the cilia appear to com- 

 mence and to cease their motions at the same moment. The 

 constancy 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 

 ph)'sical cause, dependent, however, on the life of the ani- 

 mal. 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 constitution of these 

 zoophytes, similar to that which has been proposed with re- 

 gard to trees, namely, what limits should be assigned to 

 their individuality? Is the whole mass, which appears ta 

 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 aggrega- 

 tion of smaller individuals: each individual being characte- 

 rized by having a single mouth, with its accom.panying ten- 

 tacula, and yet the whole being animated by a common 

 principle of life and growth? The greater number of natu- 

 ralists 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 contributing its quota to the ge- 

 neral nourishment of this animal republic. As the deter- 

 mination of this question involves tlie consideration of the 

 function of nutrition, I shall postpone its farther discussion 

 to a future part of this treatise. As far, indeed, as regards 

 the mechanical condition of animals which are so complete- 

 ly stationary, it matters little, whether the whole mass be 

 regarded as one individual animal, or as an aggregate of dis- 

 tinct individuals. But the question becomes of some impor- 

 tance when applied to detached zoophytes, such as Fenna- 

 iiila, which are formed of a multitude of polypes connected 

 with a common stem, but w^hich float at liberty in the sea. 



