he posterior extremity forms the enlarged
funnel-like branchial or cloacal chamber. The anterior extremity is
also somewhat enlarged. The whole surface is uniformly covered with
short compressed calcareous spicula embedded in the cuticle.

[Illustration: FIG. 10.--_Chaetoderma nitidulum_, Loven (after Graff).
The cephalic enlargement is to the left, the anal chamber (reduced
pallial chamber, containing the concealed pair of ctenidia) to the
right.]

_Branchiae._--The single pair of branchiae are placed symmetrically
right and left of the anus, and each has the structure of a ctenidium
bearing a row of lamellae on each side as in the Polyplacophora.

_Intestine._--The mouth is anterior, terminal and crescentic, and
beneath it is a rounded ventral shield. On the floor of the pharynx or
buccal mass is a rudimentary radula, which in many species consists of
a single large tooth, bearing two small teeth or a row of teeth. In
other species the radula is more of the usual type consisting of
several transverse rows of two or three teeth each. Two pairs of
salivary glands open into the buccal cavity. The digestive tube is
straight and simple, wider in its anterior part, into which opens the
duct of the hepatic caecum (fig. 4, B). The latter extends backwards
on the ventral side of the intestine.

_Coelom, Gonads and Excretory Organs._--These are closely similar in
their relations to those of the Neomeniomorpha. The chief difference
is that the gonad or generative portion of the coelom is single and
median, opening into the pericardium by a single posterior aperture.
The excretory organs or coelomoducts arise from the posterior corners
of the pericardium, run forwards and then backwards to open by
separate apertures lateral to the gills (fig. 5, A). There are no
accessory generative organs.

_The heart and vascular system_ are similar to those of the
Neomeniomorpha, the only important differences being that the
ventricle is nearly free in the pericardial cavity, and that the
latter is traversed by the retractor muscles of the gills.

_Nervous System._--There are two closely connected cerebral ganglia,
from which arise the usual two pairs of nerve cords. Pallial and pedal
on each side are closer together than in the other groups, and
posteriorly they unite into a supra-rectal cord provided with a median
ganglionic enlargement (fig. 7, C). A small stoma