groups can be
brought into line with respect to the metamerism of the head, the
position of the generative orifices will remain as a fundamental and
constant character, distinguishing the Chilopoda from the other groups
of so-called "Myriapods" and the Hexapoda from the Symphyla, which in
many particulars they resemble.

_Structure of the Chilopoda._--The exoskeletal elements of a typical
somite consist of a dorsal plate or tergum, a ventral plate or
sternum, a lateral or pleural membrane, often strengthened with
chitinous sclerites, and a pair of appendages. At the anterior
extremity there is a head-shield or cephalite, which bears eyes, when
present, and a pair of antennae. In all centipedes, except the
_Scutigeridae_, the preantennal portion of the cephalite is sharply
reflexed, ventrally forming an area called the clypeus. The inferior
edge of this bears the labrum, which is usually represented by a small
median, and two large lateral plates. The appendages are modified as a
single pair of antennae, four pairs of jaws or gnathites, a variable
number of walking legs and a single pair of generative limbs or
gonopods. The antennae, articulated to the forepart of the head and
preoral in position, are long and flexible and consist of fourteen or
more segments. The jaws of the first pair of mandibles are stout and
bi-segmented, with a dentate cutting edge. Those of the second pair or
maxillae vary considerably in structure in different groups. They are
foliaceous and are usually regarded as biramous. In some genera
(_Scutigera_, _Lithobius_) the inner branch consists of two distinct
segments meeting those of the opposite side in the middle line. The
outer branch, which is always larger, consists of three or four
segments. Generally, however, the basal segments of the two branches
are coalesced with each other and with the corresponding segments of
the opposite side to form a single broad transverse plate. The above
described condition seen in _Scutigera_ suggests that two pairs of
jaws may be involved in the formation of the maxillae in the
Chilopoda. The jaws of the third pair, the palpognaths or second pair
of maxillae, resemble dwarfed walking legs, and consist of five or six
segments, of which the basal or coxa is united mesially to its fellow.
The jaws of the fourth pair, the toxicognaths or poison-jaws, are long
and powerful, and consist like the legs pri