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 permanently preserved as fossils in this defensive
condition.
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