. 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. 31 g),
the animal was blind. The lateral portions of the head-shield
are usually separated from the central portion by a peculiar
line of division (the so-called "facial suture") on each side;
but this is also wanting in some of the Cambrian species. The
backward angles of the head-shield, also, are often prolonged
into spines, which sometimes reach a great length. Following
the head-shield behind, we have a portion of the body which is
composed of movable segments or "body-rings," and which is
technically called the "thorax," Ordinarily, this region is strongly
trilobed, and each ring consists of a central convex portion,
and of two flatter side-lobes. The number of body-rings in the
thorax is very variable (from two to twenty-six), but is fixed
for the adult forms of each group of the Trilobites. The young
forms have much fewer rings than the full-grown ones; and it
is curious to find that the Cambrian Trilobites very commonly
have either a great many rings (as in _Paradoxides_, fig. 31,
a), or else very few (as in _Agnostus_, fig. 31, g). In some
instances, the body-rings do not seem to have been so constructed
as to allow of much movement, but in other cases this region of
the body is so flexible that the animal possessed the power of
rolling itself up completely, like a hedgehog; and many individuals
have been p