ing 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.