Savory - Arachnida 1977

The chief external feature of Arachnida is the division of the body into
two parts, properly to be called the prosoma and the opisthosoma. The
former, composed of a united head and thorax, is often called the
cephalothorax; the latter is known as the abdomen. The objection to the
use of these terms is that they are also in use for other animals, in which
their constitution is not the same as in Arachnida. The prosoma of an
arachnid appears to be made up of at least six somites, with the acron
indistinguishably fused to the first; and each somite carries a pair of
appendages: chelicerae, pedipalpi or legs, which are present in all
normal Arachnida without exception. The dorsal plates or tergites of
the prosoma are usually fused to form a carapace, which may be uniform
and almost free from traces of its segmental origin, but in some
orders the posterior tergites are separate. The sternites of the lower
surface show a considerable variation.
In the opisthosoma a maximum of 13 somites is recognizable.
The first of these may be present only in the embryo and be missing
from the adult body. In some orders there is a distinction between a
mesosoma of seven and a metasoma of five somites: in others a few
posterior somites may form a pygidium. A telson may or may not be
present. In general the opisthosoma carries no appendages, the spinnerets
of spiders being an obvious and familiar exception.
The subject of the segmentation of the arachnid body is one of some
uncertainty and difference of opinion, and it is probable that the
account given here may need correction in the future.
At the front, the primitive acron is seldom recognizable. If it persists at
all it is so closely fused with the first or cephalic somite that no distinction
remains. Belief in a rostral somite behind it is no longer maintained, and
it is usual to regard the head as composed of a single somite only, with
the chelicerae as its appendages. These are the only appendages in
front of the mouth, which during development moves backwards from
its primitive anterior position, so that the chelicerae, formerly be~·-·~
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