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 ¢erra de sienna 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. 
MADREPORIDA, 
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 laminz 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 
which 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 subtentacular 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 Madreporide 
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 peculiarity of structure in order to divide the Madre- 
pores into five sections—namely, MJadrépores apores, Madrépores per- 
torés, Madrépores tabulés, Madrépores tubulés, and Madrépores rugueux. 
In the group of Aporous Maprepores, 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- 
