NATURAL HISTORY. 
ROOM III.] 
it has very generally been confounded. The animals 
are united together in similar common masses ; but 
they are much more complicated in their organization, 
have a regular digestive canal, a separate stomach, 
and distinct mouth and vent. Their mouth is surround¬ 
ed with eight or more simple ciliated tentacles, which, 
when the animal is at rest, are contractile into the gul¬ 
let. Their outer skin is in general thick or fleshy, and 
very often assumes a stone-like or glassy consistence. 
They are nearly allied to the compound Ascidia in 
their organization, but are distinguished from them by 
their mouth being furnished with distinct tentacles, and 
by their skin generally assuming gradually a stony 
texture. 
In the fluviatile kinds, the series of tentacles is inter¬ 
rupted on one side, so that they form a horse-shoe 
shaped group, as in Plumatella , which is found in the 
ponds and ditches near London. 
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 Alcyonidium , 
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 Sertidaria , 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 formed it increase in age; the skin 
