of which ulterior
divisions are only slight modifications founded on the development or
addition of some parts that do not produce any essential change of
structure.

The four great branches of the animal world are the _vertebrata_,
_mollusca_, _articulata_, and _radiata_. The _vertebrata_ are those
animals which (as man and other sucklers, birds and fishes) have a
backbone and a skull with lateral appendages, within which the viscera
are excluded, and to which the muscles are attached. The _mollusca_ or
soft animals have no bony skeleton; the muscles are attached to the
skin, which often include stony plates called shells; such mollusca are
shell-fish, others are cuttle-fish, and many pulpy sea animals. The
_articulata_ consist of crustacea (lobsters, &c.), insects, spiders, and
annulos worms, which, like the other classes of this branch, consist of
a head and a number of successive portions of the body jointed together,
whence the name. Finally the _radiata_ include the animals known under
the name of zoophytes.

Now it is fossils of the _radiata_ division of the animal kingdom that
are found in the lowest stratified rocks, polypiaria and crinodia, the
first including various forms of these extraordinary animals
(corallines) which still abound in tropical seas, often obstructing the
course of the mariner, and even laying the foundation of new continents.
The crinoids are an early and simple form of the large family of
star-fishes; the animal is little more than a stomach, surrounded by
tentacula to provide itself with food, and mounted upon a many-jointed
stalk, so as to resemble a flower upon its stem. Along with these in the
slate system are a few lowly genera of crustacea, and of a higher class,
the mollusca, and the existence of these imply the contemporary
existence of certain humbler forms of life, vegetable and animal, for
their subsistence, forming a scene approaching to what is found in seas
of the present day, excepting that fishes, nor any higher vertebrata, as
yet roamed the marine wilds.

The animal species of this era seem to have been few in number, and
almost the whole had become extinct before the next group of strata had
been formed. In the Silurian deposit the vestiges of life become more
abundant, the number of species extended, and important additions made
in the traces of sea plants and fishes. Remains of fishes have been
detected in rocks immediately over the Aymestry limestone, being
appare