ON POLYPS. 



Gorgonise of the Indian Ocean. In the Isis Hippuris (jig. 7, B) 

 the central axis is alternately composed of both these substances, 

 exhibiting calcareous masses united at intervals by a flexible mate- 

 rial, allowing the stem to bend freely in every direction. The 

 object of such diversity in the texture of the polypary of the Coral- 

 hdce will be at once apparent when we consider the habits of the 

 different species : the short and stunted trunks of Corallium, 

 composed of hard and brittle Fig. 7. 



substance, are strong enough 

 to resist injuries to which they 

 are exposed ; but in the tall 

 and slender stems of Gor- 

 gonia and Isis, such brittle- 

 ness would render them quite 

 inadequate to occupy the si- 

 tuations in which they are 

 found, and the weight of 

 the waves falling upon their 

 branches would continually 

 break in pieces and destroy 

 them ; this simple modifica- 

 tion, therefore, of the nature 

 of the secretions with which 

 they build up the skeleton 

 which supports them allows, 

 them to bend under the passing waves, and secures them from 

 otherwise inevitable destruction. 



(45.) Upon making a transverse section of one of these poly- 

 paries, (Jig. 7, A,) the solid axis is distinctly seen to be made up 

 of layers arranged in a somewhat undulating manner around the 

 centre, and successively deposited by the living cortex : the growth 

 of the stem, in the harder species at least, is very slow, and several 

 years are necessary to its maturity ; a circumstance whicFlias ren- 

 dered it needful to impose strict laws, forbidding the Mediterranean 

 coral-fishers to disturb too frequently the same localities, which are 

 only visited at stated periods. 



(46.) The deposition of solid matter in the soft bodies of these 

 polyps is not confined to the production of the central stem, but in 

 many even of the Keratophyta * cretaceous particles are extensively 



* An old name for polyps with a horny axis, x'^xf, horn; Qvrov, a stem ; as distin- 

 guishing them from the stony polyps, Lithophyta, *.i0 a{ , a stone ; <f>vrav. 



D 



