the Polyplacophora
are: (1) they are worm-like in shape; (2) there is no distinct foot, and
the mantle bears no shell-valves, but only numerous calcareous spicules;
(3) the digestive tube is straight.

_Neomenia_ and its allies are marine animals living at depths of 15 to
800 fathoms on soft muddy ground; they are found crawling on corals and
hydrozoa, on which they feed. The British genera are: _Neomenia,
Rhopalomenia_ and _Myzomenia_. They have been taken in nearly all seas
except the South Atlantic and S.E. and N.W. Pacific. About forty species
are known. _Chaetoderma_, of which nine species have been described, has
similar habits and distribution, but feeds chiefly on Protozoa. The
order Aplacophora is divided into two suborders.

Suborder I. NEOMENIOMORPHA.--Aplacophora with a distinct longitudinal
ventral groove; bisexual with paired genital glands and no distinct
liver. The whole of the skin except the ventral groove corresponds to
the mantle of _Chiton_. The cuticle, in some species very thick,
contains numerous spicules which are long, hollow and calcified; they
are secreted by epithelial papillae. In some species there are also
sensory papillae comparable to the aesthetes of Chitons. A small
longitudinal projection in the ventral groove represents the foot.
Into the groove open mucous glands, a large one anteriorly and another
opening into a posteriorly cloacal, branchial cavity.

[Illustration: FIG. 9.--_Neomenia carinata_, Tullberg (after Tullberg).

A, Lateral view.
B, Ventral view.
C, Dorsal view.
D, Ventral view of a more extended specimen.
a, Anterior.
b, Posterior extremity.
c, Furrow, in which the narrow foot is concealed.]

_Branchiae._--In _Neomeniidae_ and most of the _Parameniidae_ there is
a circlet of gills on the inner walls of the cloacal chamber. These
gills are simple folds or laminae of the body wall. In other species
they are absent.

_Intestine._---The mouth opens into a muscular pharynx lined by a
thick cuticle. Into the pharyngeal cavity open salivary glands and
radular sac. The former are paired and ventral, and open on a
subradular prominence. In some species there is a second dorsal pair.
_Neomenia_ and other genera have no salivary glands.

The radula when present comprises several transverse rows of teeth,
and each transverse row may have several teeth (polystichous), two
teeth (distichous), or one