.
As in other molluscs, the arteries do not extend far, but lead into
inter-visceral blood-spaces. The venous blood is conducted from
the tissues to a large sinus on either side above the pallial groove,
and from this sinus passes to the gills by an afferent vessel in each
gill on the internal or pedal margin of the axis. The oxygenated
blood is carried from each gill by an efferent vessel on the external
or pallial side of the axis to another longitudinal vessel which leads
to the auricle on each side.

[Illustration: After Haller (_Arbeiten zool. Instit._), Vienna, 1882.

FIG. 6.--Dissection of the renal organs (nephridia) of _Chiton siculus._

F, Foot.
L, Edge of the mantle not removed in the front part of the specimen.
s.o., Oesophagus.
af, Anus.
gg, Genital duct.
go, External opening of the same.
eg, Stem of the nephridium leading to no, its external aperture.
nk, Reflected portion of the nephridial stem.
ng, Fine caeca of the nephridium, which are seen ramifying
transversely over the whole inner surface of the pedal muscular
mass.]

_Nervous System._--There are no well-marked specialized ganglia in the
central nervous system, nerve-cells being distributed uniformly along
the cords. There are two pairs of longitudinal cords, a pedal pair
situated ventrally and united beneath the intestine by numerous
commissures, and a pallial pair situated laterally and continuous with
one another above the rectum (fig. 7). The four cords are all
connected anteriorly with the cerebral commissure which lies above the
buccal mass anteriorly. From the points where the cords meet the
cerebral commissure, arise on each an anterior labial commissure and a
stomatogastric commissure. The letter bears two ganglion swellings,
the buccal ganglia. The labial commissure gives off a subradular
commissure which also bears two ganglia, these being in close relation
to a special sense-organ called the subradular organ, an epithelial
projection with nerve-endings, lying in front of the radula and
probably gustatory in function. One osphradium or branchial olfactory
organ is usually present on each side, on either side of the anus on
the inner wall of the mantle, near the base of the last gill. In
_Lepidopleuridae_ an osphradium occurs at the base of each gill. The
sense organs of the shell-valves have already been described.