70 NATURAL HISTORY. |~N. ZOOL. GAL. 
taneously, the collars of the different tubes come in con¬ 
tact, and, having chalky matter deposited in them, unite 
and at length become stony like the rest of the tube. 
After this has been done, the animal begins to form 
another tube ; and after a time they repeat the same 
process again, so that the different tubes of the same 
group are united by cross layers into a single mass. The 
animal being bright green, and the coral vivid red, makes 
it a most beautiful object when alive. 
By far the greater number of the animals of this order 
have a very thick, spongy outer skin, which is often 
strengthened by having variously shaped calcareous 
grains, or rugose and more or less fusiform calcareous 
spiculae, imbedded in its surface. These animals live in 
societies closely united in a single mass by their outer 
skin, while there appears to be also a general community 
of function in their more important internal organs. 
They are constantly emitting buds from different parts 
of their surface, and as each species emits them in a pe¬ 
culiar form, the mass assumes a definite shape, varying in 
the different kinds. 
In general, the common mass has an expanded base 
by which it is attached to some marine body, and when 
the mass is of a low rounded shape, as in Lobularia , (Case 
28,) the spiculae in the substance of the skin are strong 
enough to support the mass, but when the mass assumes 
an erect or a branched tree-like form, the animals secrete 
in the centre of their body a more or less rigid support, 
which has been called their axis, and which has some¬ 
times, though erroneously, (from its being commonly seen 
in collections without the remains of the investing animal,) 
been considered the entire coral. This axis is thickened 
by depositions of fresh layers of horny matter on its 
surface as the mass increases in size and requires more 
support, the increase of the thickness and length of the 
axis being always simultaneous with the growth of the 
mass. In some kinds, the axis is only formed of an im¬ 
mense number of spiculae, like those in the substance of 
the skin, being crowded together in the centre, as in the 
genus Briareum (Case 28). 
In general it is formed of a quantity of horny matter, 
which is deposited in successive layers, as in Gorgonia 
