PENNATULA. 



157 



tributing its quota to the general nourishment of 

 this animal republic. As the determination of this 

 question involves the consideration of the function 

 of nutrition, I shall postpone its further discussion 

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

 regards the mechanical condition of animals which 

 are so completely stationary, it matters little 

 whether the M'hole mass be regarded as one in- 

 dividual animal, or as an aggregate of distinct 

 individuals. But the question becomes of some 

 importance when applied to detached zoophytes, 

 such as PenfiatidcE, which are formed of a multi- 

 tude of polypes connected with a common stem, 

 but which float at liberty in the sea. 



The Pennatula (Fig. 71) has been termed the 



sea pen, from the circumstance 

 of its calcareous axis, or stem, 

 having a double set of branches, 

 extending in the same plane from 

 both the sides, like the vane of a 

 quill, and of its series of polypes 

 being set along one edge of each 

 branch, like the filaments which 

 arise from the fibres of the fea- 

 ther. Some of these polypes are 

 seen magnified in Fig. 72. Immense numbers of 

 these curious animals are met with in different 

 parts of the ocean. If they possessed in any degree 

 the power of locomotion, which many naturalists 

 have ascribed to them, we should be able to ascer- 

 tain whether all their movements are conducted by 

 a common volition, or whether they are performed 

 independently of one another. It has often, indeed, 

 been asserted, that pennatulae swim through the 



