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.
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