46 



ON POLYPS. 



(65.) It has been generally stated that the living pith exuded from 

 its surface the horny matter which, by its concretion, forms the 

 tube or external skeleton investing the whole ; the accuracy of such 

 a supposition, however, may well be questioned. We have already 

 seen, in the Tubipora musica, that the calcareous tube investing that 

 polyp was produced by the interstitial deposit of earthy matter in 

 the membrane which formed originally its outer case. In the tribe 

 of zoophytes which we are now speaking of, we shall find the 

 exterior tube to be formed in a way precisely similar. On referring 

 to the diagram, (fig. 14,) the mode of its growth will be rendered in- 

 telligible: the soft part or living axis of the polypary is seen to be 

 contained in two distinct layers ; the inner one composing the 

 digestive sac of the polyp, and embracing the granular matter, 

 which seems to be the special seat of the nutritive process ; the 

 outer or tegumentary layer, i, after leaving the tentacula, may 

 be traced down the sides of each polyp to the bottom of the cell, 

 where its course is arrested by a slight partition, at which point 

 it turns outwards, lining the interior of the cell as far as its margin, 

 where, as in the Tubipora, it is seen to be continuous with the 

 horny matter itself. It is this tegumentary membrane, then, which 

 forms by its develope- ji. 14. 



ment the entire skele- 

 ton : as it expands, it 

 gives origin to the cells 

 and branches character- 

 istic of the species; and, 

 from being at first quite 

 soft and flexible, it gra- 

 dually acquires hardness 

 and solidity by the de- 

 position of corneous 

 matter in its sub- 

 stance. 



The cells thus formed 

 are inhabited by polyps 

 analogous to those which 

 provide nourishment for 

 the cortical families ; but 

 differing in the number 

 and appearance of the 

 tentacula, which are 



