A, Brain of _Scolopendra_. n.ant, Antennal nerves; n.opt, ocular
nerves; n.pr.ant, preantennal nerves; oes. comm, oesophageal
commissure.
B, Section of Eye of Scolopendra. len, Corneal lens; ret, retinal or
visual cells; n.opt, optic nerve.
C, Ocular unit or ommatidium of compound Eye of _Scutigera_. len,
corneal lenticle; c.c, crystalline cone; 1, pigmented cells of
outermost tier; 2, 3, retinular cells of middle and innermost tiers;
rbd, rhabdomeres; n.opt, optic nerve; pg, pigment cells.]
The alimentary canal is a simple tube running without convolutions
from the mouth to the anus. Its anterior portion or pharynx, which
arises from the stomodaeal invagination in the embryo, is short; a
pair of large, so-called salivary glands open into it. The mesenteric
part of the canal is relatively wide and receives at its junction with
the hind-gut the excretory products of a pair of very long and slender
malpighian tubes of proctodaeal origin. The posterior end of the
canal, arising from the proctodaeum, is relatively short and narrow.
[Illustration: FIG. 3.--Diagram of Alimentary Canal of _Lithobius_.
a, Anus.
mg, Mid-Gut.
hg, Hind-Gut.
mt, Malphighian tubule.
s.gl, Salivary gland.
lg. 1, lg. 15, Legs of first and fifteenth pairs.]
The generative organs vary in structural details in different
centipedes. In the male of _Lithobius_ the testes consist of a single
coiled tube lying above the alimentary canal. The slender vas deferens
which proceeds from its hinder end divides posteriorly into a right
and left branch, embracing the gut and uniting beneath it to form a
common chamber or atrium within the genital orifice. The atrium
receives the secretion of two pairs of large accessory glands; and a
pair of tubes, or vesiculae seminales, open, one on each side, into
the divided sperm ducts close to their point of origin above the
intestine. The organs of the female are very similar. There is a large
median ovary followed by a short oviduct forming a circum-intestinal
collar and a common atrium. Into the latter open a pair of short
receptacula seminis and the slender duct of two pairs of large
accessory glands. There is nothing in the female corresponding to the
supra-intestinal vesiculae seminales of the male. In the male of
_Scolopendra_, on the contrary, there are as many as twelve pairs of
somewhat sausage-shaped testes, approximated two by two. From each
pair proceed two slender ducts which open into a median duct coiled in
the posterior third of the body and much expanded in the last three of
the leg-bearing segments. The right and left portions of the
intestinal ring of the genital duct are unequally developed, and there
are no vesiculae seminales, but two pairs of accessory glands
communicate with the genital atrium as in _Lithobius_. In the female
_Scolopendra_ the right and left portions of the intestinal collar are
also unequally developed, and only a single pair of accessory glands
besides the receptacula seminis 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.