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CHAPTER V. 

 Functions and Instincts. Polypes. 



THE tribe of animals to which we are next to 

 direct our attention, though not invisible like 

 the last, are almost equally concealed from our 

 view by the medium that they inhabit ; so that, 

 with the exception of those that abound in fresh 

 water, and are easily kept alive for examination, 

 the great body of them inhabiting the ocean, 

 can seldom be studied in a living state. All the 

 polypes are aggregate animals, in which they 

 differ from the majority of the preceding class. 

 The most imperfect of them, as the sponges and 

 some of the alcyons, seem to consist merely of 

 a gelatinous mass, without any organs of pre- 

 hension, which by its alternate contraction and 

 dilatation, imbibes or sends out the water from 

 which the animal derives its nutriment ; but the 

 great majority have a mouth furnished with 

 arms or tentacles varying in number. These 

 are described as tubes, filled with fluid, expand- 

 ing at the base into a small cavity, which when 

 contracted necessarily propels the fluid into the 

 tentacles, and thus extends them ; but when 



