r by means of spiracles
perforating the lateral or pleural membrane of more or fewer of the
somites below the edge of the terga. Spiracles are never present upon
the anal, genital and last leg-bearing somites, and only rarely, as in
_Henicops_, upon the somite bearing the legs of the first pair. In the
majority of cases the spiracles are circular, sigmoid or slit-like
orifices, with chitinized rim, leading into a pocket-like integumental
infolding, from which emanate numerous small tracheal tubes which soon
anastomose to form the main tracheal trunks. In _Dacetum_, one of the
_Scolopendridae_, there is no pocket-like infolding, the small
tracheal tubes opening direct to the exterior on a large subcircular
plate where their apertures fuse to form a complicated network. The
apertures, as in the case of other genera, are protected by fine
hairs; and the tracheae themselves are strengthened by a fine spiral
filament. In the _Lithobiidae_ the tracheae do not anastomose; but in
_Scolopendra_ and _Geophilus_ the main trunks in each segment fuse
transversely with those of the opposite side and also longitudinally
with those of the preceding and succeeding segments.

In _Scutigera_ the tracheae differ both in structure and position from
those of all other Chilopoda. The spiracles, unpaired and seven in
number, open in the median dorsal line. Each leads into a short sac
from which five tracheal tubes depend into the pericardial
blood-sinus.

Existing Chilopoda may be classified as follows, into five orders
referable to two subclasses--

Subclass I. Pleurostigma.
Order 1 Geophilomorpha.
Order 2 Scolopendromorpha.
Order 3 Craterostigmomorpha.
Order 4 Lithobiomorpha.

Subclass II. Notostigma.
Order 5 Scutigeromorpha.

SUBCLASS I, PLEUROSTIGMA.--Chilopods furnished with a rich system of
branching tracheal tubes, the spiracles of which are paired and open
upon the pleural area of more or fewer of the somites. Each leg-bearing
somite contains a distinct tergum and sternum, the number of sterna
never exceeding that of the terga. Eyes are either preserved or lost;
when preserved they are represented either by a single one-lensed
ocellus or by an aggregation of such ocelli on each side of the head.
The anterior portion of the head, bearing the labrum, is bent sharply