SECT. IT. TEXT A CLES OF A CTIX1JE. 1 3 7 



most prominent ; and sometimes the edges of the lamellae 

 rise high above the hollow or cup which is the centre 

 of the star, and contains the mouth of the polype. In 

 some families of corals these edges, which form the rays, 

 are toothed or spined. 3 



A horny column in the axis of the polype, hardened 

 by sulphate and carbonate of lime, and called the 

 coluinella, generally shows its top in the centre of the 

 star, and varies in structure in the different genera. 

 Thus the Actinian polypes may be said to possess an 

 internal skeleton, and as they approach maturity they 

 also acquire an external one in the form of a cylindrical 

 coat, or stony wall, which surrounds them, and into which 

 most of them can withdraw the soft upper part of their 

 bodies and tentacles, so as to be partly or altogether con- 

 cealed. The perpendicular lamellae are sometimes ex- 

 tended through the stony walls of the polype, so as to 

 form a series of broad, well-developed ribs on its exterior 

 surface. 



The stony substance of corals is chiefly carbonate of 

 linie, which the polypes have the power of abstracting 

 from the sea-water, combined with a small quantity of 

 animal matter, and a still smaller quantity of phosphate 

 of lime, with a trace of silver and magnesia. This stony 

 substance takes the crystalline form of needles. By the 

 successive deposition of these needles, a network is 

 formed round the body of the animal, which by a series 

 of these deposits is condensed into a hard impervious 

 coat or wall. During this formative process many cha- 

 racteristic forms may be produced by division and build- 

 ing, depending upon the genus and species of the polype ; 

 but they do not lay eggs till they come to maturity. 



Some corals increase both by budding and division, 

 but by far the greater number grow in size by budding, 



8 ' Histoire des Corallines,' par Professeur Milne-Edwards. 



