POLYPI. 163 



warmer regions of the globe. Their flesh ex- 

 hibits the same granular appearance as that of 

 the sponge, but it is generally firmer, and often 

 intermingled with masses of calcareous matter. 

 The tentacula, which may be compared to arms, 

 vary in number and in length in different spe- 

 cies of polypi, and sometimes, instead of a single 

 row, each of the mouths has two or more series 

 of tentacula placed around it. They are formed 

 of a prolongation of the soft substance of the 

 polypus, and are sometimes tubular; and their 

 cavities are then continuous with that of the 

 general internal cavity into which the several 

 mouths open. Besides being flexible in every 

 direction, the tentacula are also capable of being 

 lengthened or shortened at the pleasure of the 

 animal. Their elongation is produced by the 

 propulsion of a fluid into their interior, derived 

 from the general cavity of the body; and their 

 retraction is effected by the return of the same 

 fluid. 



The whole arrangement of the tentacula on 

 the margin of the projecting mouths bears a 

 striking resemblance to a flower, especially to 

 those which, like the daisy, or china-aster, have 

 the corolla composed of slender radiating petals. 

 We find, indeed, tliat as the organs of zoophytes 

 become more developed, the affinities which these 

 lower departments of the animal kingdom retain 

 with plants, are more marked and more predo- 



