ermanently preserved as fossils in this defensive
condition. Finally, the body of the Trilobite was completed by
a tail-shield (technically termed the "pygidium"), which varies
much in size and form, and is composed of a greater or less number
of rings, similar to those which form the thorax, but immovably
amalgamated with one another (fig. 31, h).

The under surface of the body in the Trilobites appears to have
been more or less entirely destitute of hard structures, with the
exception of a well-developed upper lip, in the form of a plate
attached to the inferior side of the head-shield in front. There
is no reason to doubt that the animal possessed legs; but these
structures seem to have resembled those of many living Crustaceans
in being quite soft and membranous. This, at any rate, seems to
have been generally the case; though structures which have been
regarded as legs have been detected on the under surface of one
of the larger species of Trilobites. There is also, at present,
no direct evidence that the Trilobites possessed the two pairs
of jointed feelers ("antennae") which are so characteristic of
recent Crustaceans.

The Trilobites vary much in size, and the Cambrian formation
presents examples of both the largest and the smallest members
of the order. Some of the young forms may be little bigger than
a millet-seed, and some adult examples of the smaller species
(such as _Agnostus_) may be only a few lines in length; whilst
such giants of the order as _Paradoxides_ and _Asaphus_ may reach
a length of from one to two feet. Judging from what we actually
know as to the structure of the Trilobites, and also from analogous
recent forms, it would seem that these ancient Crustaceans were
mud-haunting creatures, denizens of shallow seas, and affecting
the soft silt of the bottom rather than the clear water above.
Whenever muddy sediments are found in the Cambrian and Silurian
formations, there we are tolerably sure to find Trilobites, though
they are by no means absolutely wanting in limestones. They appear
to have crawled out upon the sea-bottom, or burrowed in the yielding
mud, with the soft under surface directed downwards; and it is
probable that they really derived their nutriment from the organic
matter contained in the ooze amongst which they lived. The vital
organs seem to have occupied the central lobe of the skeleton,
by which they were protected; and a series of delicate leaf-like
paddles, which probabl