NATURAL HISTORY. 
147 
N. ZOOL. GAL.] 
attached to leaves which rot every year, the animal only 
forms a thin layer of branching tubes, but when attached 
to permanent bodies, as twigs, bricks, &c., they form 
thick masses. 
In the marine kinds, which are far more numerous, the 
tentacles form a continuous circular series. 
These animals live united together in masses, exhibit¬ 
ing almost all the different external forms assumed by 
the preceding Zoophytes. In the genus Alcyomdium , 
the outer skin is thick and cartilaginous, forming, when 
the animals are united together, a hard fleshy mass, which 
has been mistaken for an alga, and is difficult to keep 
in a dry state. In others, the skin is more or less trans¬ 
parent and horny, so that the mass assumes much of the 
appearance of Sertularia , as in the genera Serialaria , &c., 
(Case 31,) but in general the skin is more or less rapidly 
hardened into a ston}^ case, according to the manner in 
which the animal is reproduced either by the spontane¬ 
ous division of its body, or by the emission of buds from 
various parts of its surface, and these differences produce 
very differently shaped corals. 
Sometimes the mass of animals assumes the form of a 
leaf-like expansion, attached by its lower surface to shells 
and other marine bodies ; then the younger animals are 
formed on the circumference of the mass, and if the sur¬ 
face of the coral is examined, it will shew all the extra* 
ordinary and different changes which the skin assumes as 
the animals which ormed it increase in age; the skin 
of the younger animal being thin and soft, it gradually 
becomes harder until the animal arrives at its perfect 
state of developement, and then it thickens, blisters, and 
swells until the hole through which the head of the 
animal was emitted is obliterated, and the animals are 
destroyed by the developement of their own skin. These 
cells are to be found gradually spreading from the centre 
of the expansion or the base of the stem of the more tree¬ 
like coral, while the new animals and their cells are being 
developed on the edge of the frond or the tips of the 
branches. Sometimes, as the mass increases in size, it 
sends up more or less erect lobes, and if these lobes have 
both of their sides equally exposed to the influence of 
the sea and light, so that the animals of each side can 
