174 THE MECHANICAL FUNCTIONS. 



regarding each portion, so provided with a dis- 

 tinct circle of tentacula, as a separate animal, 

 associated with its neighbours in the construc- 

 tion of a common habitation, and contributing 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 whole mass be regarded as 

 one individual animal, or as an aggregate of 

 distinct individuals. But the question becomes 

 of some importance when applied to detached 

 zoophytes, such as Petmatula, which are formed 

 of a multitude of polypes connected with a com- 

 mon 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 feather. Some of 

 these polypes are seen magnified in Fig. 72. 

 Immense numbers of these curious animals are 



