This shell (fig. 31) generally exhibits a very
distinct "trilobation" or division into three longitudinal lobes,
one central and two lateral. It also exhibits a more important and
more fundamental division into three transverse portions, which
are so loosely connected with one another as very commonly to be
found separate. The first and most anterior of these divisions
is a shield or buckler which covers the head; the second or middle
portion is composed of movable rings covering the trunk ("thorax
"); and the third is a shield which covers the tailor "abdomen." The
head-shield (fig. 31, e) is generally more or less semicircular
in shape; and its central portion, covering the stomach of the
animal, is usually strongly elevated, and generally marked by
lateral furrows. A little on each side of the head are placed
the eyes, which are generally crescentic in shape, and resemble
the eyes of insects and many existing Crustaceans in being
"compound," or made up of numerous simple eyes aggregated together.
So excellent is the state of preservation of many specimens of
Trilobites, that the numerous individual lenses of the eyes have
been uninjured, and as many as four hundred have been counted
in each eye of some forms. The eyes may be supported upon
prominences, but they are never carried on movable stalks (as
they are in the existing lobsters and crabs); and in some of the
Cambrian Trilobites, such as the little _Agnosti_ (fig.
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