42 THE AMPHINEURA 
all around the latter there is a pallial groove. The mantle secretes 
a shell consisting of eight plates or valves articulated with one 
another and arranged in longitudinal series. Each of these valves 
partially overlaps the following, except in some species of Cryptoplax 
( = Chitonellus), in which the three hindermost are isolated. This 
articulation of the valves allows the animal to roll up. The two 
terminal (first and eighth) valves are semicircular, the six inter- 
mediate are quadrangular. They may be partially (Cryptoplax and 
some species of Acanthochiton) or even wholly (in adult Cryptochiton, 
but not in young ones) concealed by a redupli- 
cation of the mantle. 
Each valve is made up of two quite dis- 
similar calcareous layers: («@) the uppermost or 
tegmentum, which alone is visible externally ; 
(b) the deeper layer or articulamentum, which 
is porcellaneous, quite compact, and invisible 
in the living animal. In most of the lower 
Polyplacophora these layers are coextensive 
and have smooth edges, but in the higher 
forms the articulamentum projects beyond the 
-* outer layer into the substance of the mantle, 
Gray dorsal aspech shew. to which it is firmly attached. These pro- 
aoe ee tel eree uz jections of the outer or peripheral margins 
first. eyes on the second of the valves are termed ‘insertion plates”; 
shell- plate; ILI, third : 
shell-plate. they are generally slit or notched to form the 
so-called ‘“ teeth,” which may be either smooth 
and sharp along the edge or crenulated. The anterior margin of 
each valve, except the first, is invariably provided with two pro- 
jections called ‘ sutural laminae,” which underlie the hind margin of 
the valve next in front. 
The tegmentum has no representative in the shells of other 
Mollusca. It is formed by the fold of the mantle covering the 
edge of the articulamentum, and, as it grows in width, it extends 
over the latter. It is much reduced in Acanthochiton and aborted 
in the adult Cryptochiton. The stratified layers of the tegmentum are 
traversed by a system of numerous, nearly parallel, ramified canals 
through which special sense-organs pass to the surface (Fig. 24). 
Nearly the whole of the peripheral part of the mantle or 
‘oirdle,” as it is called, is covered with chitinous or calcareous 
spicules of various shape, acicular or squamose. Lach spicule rests 
on an epidermic papilla and is formed by a single matrix cell. 
The head is more or less cylindrical, consisting of a short down- 
wardly curved snout with the mouth at its extremity. On either 
side of the mouth is a somewhat angular labial palp. A narrow 
furrow separates the head from the foot. The latter forms a 
ventral creeping surface, extending the whole length of the body 
Fic. 23. 
‘ 
