[Illustration: FIG. 1.--Three views of Chiton.
A. Dorsal view of _Chiton Wosnessenksii_, Midd., showing the eight
shells. (After Middendorf.)
B. View from the pedal surface of a species of Chiton from the Indian
Ocean, _p_, foot; _o_, mouth (at the other end of the foot is seen the
anus raised on a papilla); _kr_, oral fringe; _br_, the numerous
ctenidia (branchial plumes); spreading beyond these, and all round the
animal, is the mantle-skirt. (After Cuvier.)
C. The same species of Chiton, with the shells removed and the dorsal
integument reflected, _b_, buccal mass; _m_, retractor muscles of the
buccal mass; _ov_, ovary; _od_, oviduct; _i_, coils of intestines;
_ao_, aorta; _c'_, left auricle; _c_, ventricle.]
[Illustration: FIG. 2.--Pallial eye and aesthetes of _Acanthopleura
spiniger_ (Moseley).]
Each of the eight valves of the shell is made up of two distinct
calcareous layers: (a) an outer or upper called the tegmentum, which is
visible externally; (b) a deeper layer called articulamentum which is
porcellaneous, quite compact, and entirely covered by the tegmentum. In
the lower forms the two layers are coextensive and have smooth edges,
but in the higher forms the articulamentum projects laterally beyond and
beneath the tegmentum into the substance of the mantle. These
projections are termed insertion plates; they are usually slit or
notched to form teeth, the edges of which may be smooth and sharp, or
may be crenulated. The anterior margin of each valve except the first is
provided with two projections called sutural laminae which underlie the
posterior margin of the preceding valve.
[Illustration: From Lankester, _Treatise on Zoology_.
FIG. 3.--Ventral aspect of three species of Polyplacophora showing
position of gills.
A. _Lepidopleurus benthus_.
B. _Boreochiton cinereus_.
C. _Schizochiton incisus_. _a_, anus; _f_, foot; _g_, gills; _m_,
mouth; _pa_, mantle; _pa'_, anal lobe of mantle; _ps_, pallial slit;
_te_, pallial tentacles.]
The tegmentum is formed by the fold of mantle covering the edge of the
articulamentum, and extends over the latter from the sides. It is the
first part of the shell formed in development. The tegmentum is much
reduced in _Acanthochiton_, and absent in the adult _Cryptochiton_. The
tegmentum is pierced by numerous vertical ramified canals which contain
epithelial papillae of the epidermis. These papillae form pallial
sense-organs, containing nerve-end bulbs, covered by a dome of cuticle,
and innervated from the pallial nerve-cords. 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, Ovary.
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.
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.