They are termed according
to their size, micraesthetes and megalaesthetes. In the common species
of _Chiton_ and many others of the family _Chitonidae_ the
megalaesthetes are developed into definite eyes, the most complicated of
which have retina, pigment within the eye, cornea and crystalline lens
(intra-pigmental eyes) (fig. 2). The eyes are arranged in rows running
diagonally from the median anterior beak of each valve to its lateral
borders There may be only one such row on either side, or many rows. In
some species the total number present amounts to thousands.

[Illustration: FIG. 4.--Diagrams of the alimentary canal of Amphineura
(from Hubrecht).]

A. Neomenia and Proneomenia.
B. Chaetoderma.
C. Chiton.
_o_, Mouth.
_a_, Anus.
_d_, Alimentary canal.
_l_, Liver (digestive gland).]

_Branchiae._--The series of gills may extend the whole length of the
body in the pallial groove, or may be confined to the posterior end.
Each gill has the structure of a typical molluscan ctenidium,
consisting of an axis bearing an anterior and posterior row of
filaments or lamellae. The gills are thus metamerically repeated;
there may be from four to eighty pairs, but there is often a numerical
asymmetry on the two sides. The largest pair of branchiae is placed
immediately behind the renal openings and corresponds to the single
pair of other molluscs, the organs being repeated anteriorly only
(Metamacrobranchs) or anteriorly and posteriorly (Mesomacrobranchs).

_Intestine._--The digestive tube in the Polyplacophora, which are
herbivorous, is longer than the body, and thrown into a few coils, the
anus being median and posterior. The mouth leads into the buccal
cavity, on the ventral side of which opens the radular caecum. Each
transverse row of teeth of the radula contains 17 teeth, one of which
is median, while the second and the fifth on each side are enlarged.
Two pairs of glands open into the buccal cavity, and at the junction
of pharynx and oesophagus is another pair called the sugar glands. The
stomach is surrounded by the liver or digestive gland, consisting of
two lobes which are symmetrical in the young animals, but in the adult
the right lobe is anterior and smaller.

[Illustration: FIG. 5.--Diagrams of the excretory and reproductive
organs of Amphineura (after Hubrecht).

A, Chaetoderma.
B, Neomenia.
C, Proneomenia.
D, Chiton.
O, Ov