ary.
P, Pericardium.
N, Nephridium.
u, External aperture of nephridium.
g, External aperture of the genital duct of Chiton.
r, Rectum.
Cl, Cloacal or pallial chamber of Neomeniae and Chaetoderma.
Br, Ctenidia (branchial plumes).]

_Coelom, Gonads and Excretory Organs._--As in other molluscs the
coelom is represented by a large pericardial cavity, situated above
the intestine posteriorly, and a generative sac which is single and
median and situated in front of the pericardium, except in the
_Nuttalochiton hyadesi_, where the gonads are in a similar position,
but are paired. The excretory organs are coelomoducts with an internal
ciliated opening into the pericardium and an opening to the exterior.
Both the openings are close together, the external opening being just
in front of the principal gill near the posterior end of the body. The
renal tube is doubled on itself, its middle part where the bend occurs
being situated more or less anteriorly. The excretory surface is
increased by numerous ramified caeca which extend beneath the body
wall laterally and ventrally, and open into the tube (fig. 6). The
sexes are distinct, and the ovary is frequently greenish in colour,
the testis red. The gonad is transversely wrinkled and lies between
the aorta and the intestine, extending from the pericardium to the
anterior end of the body. A simple gonaduct on each side arises from
the gonad near its posterior end and passes first forwards, then
backwards, and lastly outwards to the external opening in the pallial
groove, anterior to the renal aperture. There may be from one to nine
gills between the genital and renal pores.

_Heart and Vascular System._--The heart is enclosed in the
pericardium, and consists of a median elongated ventricle and a pair of
lateral auricles, so that the structure somewhat resembles that in
the Lamellibranchiata. The openings of the auricles into the
ventricle vary in different forms. In many of the lower forms
(_Lepidopleuridae, Mopalidae, Ischnochitonidae_) the opening on each
side is single and anterior. In the true _Chitonidae_ there are generally
two apertures on each side, and in two species three or four, another
instance of the tendency to metameric repetition in the group.
The auricles are connected with one another posteriorly behind the
ventricle. The ventricle leads into a single anterior median aorta