r,
flexible and annulated. The legs of the last pair are directed
backwards in a line parallel with the long axis of the body, so that
their coxae, fused in some cases with the pleural sclerites
(_Scolopendra_, _Geophilus_), or free and of large size (_Scutigera_,
_Lithobius_), serve to protect the small genital and anal somites.
They are often greatly modified. In the males of some species of
_Lithobius_ one or more of the segments is inflated or furnished with
tubercle-bearing, tactile bristles; in some Geophilomorpha the whole
limb is thickened in the male sex. In most Scolopendromorpha the basal
segment is armed beneath with spines or spikes (_Dacetum_,
_Scolopocryptops_); sometimes the whole appendage is thickened and
terminated by a sharp and serrate claw (_Theatops_, _Plutonium_). In
these cases the legs act as weapons of defence and offence. In other
cases (_Newportia_) the tarsi lose the claw, become many-jointed and
act as feelers, while in _Alipes_ the terminal segments are flattened,
leaf-like and furnished with a peculiar stridulating organ. The
genital somite is always small and sometimes retractile within the
somite bearing the last pair of legs. Its tergal plate is usually
retained, but its sternal plate is generally suppressed. In females of
the Lithobiomorpha and Scutigeromorpha the appendages of this
somite--the gonopods--are jointed, forcipate and relatively well
developed although small. In the females of the other orders they are
greatly reduced or absent. In the males their development varies
considerably. They are well developed in _Scutigera_, where they form
two pairs of digitiform sclerites, whereas in the Geophilomorpha they
are reduced to a pair of very short, two-jointed limbs. The anal
somite is always small and limbless. In _Craterostigmus_ the genital
and anal somites are represented by a pair of elongate valves
projecting between the legs of the last pair. The structure of the
gonopods is unknown, and the homology between the two valves and the
skeletal elements of the somites in question not clearly understood.

A study of the development of _Scolopendra_ has shown that the
antennae of the adult are the appendages of the second postoral
metamere and the mandibles those of the fourth, the first postoral
metamere, which has a pair of transient preantennal appendages, and
the third, which has no appendages, being exca