nis open into the atrium.

[Illustration: After Heymons, _Bibl. Zool_, 1901, by permission of E.
Nagele.

FIG. 4.--Posterior portion of generative organs of male of
_Scolopendra_ (A), of female (B). t, Testes; v.d, vas deferens; ov,
ovary; _r.s_, receptaculum seminis; gl. acc, accessory glands; g.o,
generative orifice.]

The heart is tubular and lies in the middle dorsal line immediately
beneath the integument. It consists of a series of chambers
corresponding roughly to the leg-bearing segments, and lies in a
blood-sinus formed by a pericardial membrane whence large alary
muscles extend to the sides of the body. Each chamber gives off in
_Scolopendra_ a pair of fine lateral vessels, and is furnished at its
posterior extremity with a pair of orifices by which the blood
re-enters the organ from the pericardial space. From the anterior
chamber, which lies in the first or second leg-bearing segment,
proceed three arteries, a median which runs forwards into the head to
supply the brain and other organs, and a lateral which with its fellow
of the opposite side forms an oesophageal aortic collar. From the
sides of the latter arise vessels to the gnathites, and from its
inferior portion an unpaired vessel passes forwards into the head and
another backwards above the nerve chord to the posterior end of the
body, supplying each segment in its course with a delicate lateral
branch. In _Scolopendra_ the chambers of the heart, excepting the
first and last, which are small, are subequal in size; but in forms
like _Scutigera_ where the terga are very unequal in size a
corresponding inequality in the size of the chambers is manifested.

[Illustration: A after Newport, _Phil. Trans._, 1843. B after Haase,
_Zool. Beitrage_, i. pt. 65, 1884, by permission of J.N. Kern. C after
Haase, loc. cit.

FIG. 5.

A, Anterior extremity of _Scolopendra_, showing two chambers of the
heart (h), the aortic ring (a), the alae cordis (a.m) and a cardiac
orifice (o).

B, Two segments of _Scolopendra_, showing the branching and
anastomosing tracheae and a spiracle (sp).

C, A pair of tufted tracheae of _Scutigera_. d, Dorsal plate; t.s,
tracheal sac; tr, tracheal tubes.]

In all centipedes, except _Scutigera_, respiration is effected by
chitinized tracheal tubes which extend with their ramifications
throughout the body and open to the exterio