MADREPORES. 173 



polypidom is fragile and brittle ; when dry, the branches, always 

 slender and delicate, resemble the barbs of a feather. The colour is 

 of a deep black, or rather bistre and terra de sieufta tint. Under a 

 powerful lens, the extremities of the branches appear to be covered 

 with small spines, and the stem is formed of oval and irregular con- 

 centric layers, which are so many zones of growth. Its consistence 

 is solid, so that it can be worked up and converted into chaplets for 

 pearls and other bijouterie : it is known in commerce as black coral. 



MADREPORIDiE. 



The Madrepores are better known than their congeners. They 

 are sometimes designated corals, but it must be recollected that the 

 precious coral forms no part of this group. 



The Madrepores are remarkable for the calcareous secretion which 

 always surrounds their tissue, and determines the formation of their 

 polypidom. They are in other respects easily recognised by the star- 

 like structure of their polypidom, in which may always be distinguished 

 a visceral chamber, the circumference of which is furnished with 

 perpendicular laminae or partitions, which are always directed towards 

 the axis of the body. When sufficiently developed they constitute, 

 by their assemblage, a star-like body formed of a great number of 

 rays. The polypidom is always calcareous. The consolidation of 

 the envelope of each polyp produces at first a kind of sheath, to 

 wliich Milne-Edwards has given the name of " the wall." The partitions 

 which proceed from the interior towards the axis of the visceral 

 chamber occupy the siibtentacular cells ; the terminal and open 

 portion designated the calyx is in organic continuity with the polyp, 

 which has retired thither more or less completely, as into a cell. 



Milne-Edwards remarks that the polypidom of the Madrcporidcc 

 present in their structure five principal modifications, due in part to 

 the fundamental number of which the chambered cells are the mul- 

 tiple, and in part to the mode of division of the visceral chamber, and 

 finally to the manner in which its tissue is constituted M. Edwards 

 avails himself of this pecuharity of structure in order to divide the Madre- 

 pores into five sections — namely. Madrepores apores, Madrepores per- 

 fores, Madrep07-es tabules, Madrepores tubules, and Madrepores rugueux. 



In the group of Aporous Madrepores, the polypidom is per- 

 haps the most highly organised. We find there a well-developed 

 and very perfect wall, and a well-developed visceral apparatus. The 

 calyx is symmetrically rayed ; the number of rays in the earlier stages 

 being six, which soon afterwards reaches from twelve to twenty- 



