STKUCTUKE OF COEALLID^. 



69 



Fig. 30. 



material, allowing the stem to bend freely in every direction. The ob- 

 ject of such diversity in the texture of the polypary of the Corallidse 

 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 

 substance, are strong enough to 

 resist injuries to which they 

 are exposed; but in the tall 

 and slender stems of Gorgonia 

 and Isis, such brittleness would 

 render them quite inadequate 

 to occupy the situations 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 

 modification, therefore, of the 

 nature of the secretions with 

 which they build up the skele- 

 ton which supports them al- 

 lows them to bend under the 

 passing waves, and secures 

 them from otherwise inevitable destruction. 



(150.) Upon making a transverse section of one of these polyparies 

 (fig. 30, 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 neces- 

 sary to its maturity, a circumstance which has rendered 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. 



(151.) 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 diffused through 

 the cortex, which not unfrequently is likewise gorgeously coloured by 

 secretions of different hues. In the Gorgoniae, a section of one of which 

 (Goryonia verrucosa) is represented in fig. 30, A, the earthy matter in 

 the crust is so abundant, that even when dried it will retain in some 

 measure its natural form, and exhibit the tints peculiar to the species. 



* An old name for polyps with a horny axis (/cepas, horn ; Qvrbv, a stem), as 

 distinguishing them from the stony polyps Lithophyta (\iOos, a stone; fyvrbv). 



A, transverse section of Gorgonia verrucosa. 



B, longitudinal section of Isis hippuris, exhibiting 

 the skeleton and fleshy crust. 



