CHAPTER XI. 
Molluscs,— concluded. 
Chitons Tooth-Shells, and Bivalves, —Classes Amphineura, 
Scaphopoda, and Pelecypoda. 
The Chiton Group, —Class Amphineura. 
The Molluscs included in this class may be regarded as aberrant gastropods, 
differing from ordinary forms in their symmetrical conformation, having the mouth 
and excretal orifice at the two extremities of the body, and the tissues of the mantle 
more or less spiculose. The symmetry which characterises the external parts also 
extends to the internal organisation. The group is divided into the two orders 
Polyplacophora and Aplacophora; and is regarded by some authors, and perhaps 
correctly, merely as an order of Gastropoda, and not forming a distinct class. 
Chitons, —Order Polyplacophora. 
The well-known chitons ( Chitonidce ) are the only forms included in this 
order, and are externally recognised by their shells consisting of eight separate 
pieces or valves, as 
they are termed, 
which are arranged 
over the back, and 
connected at the sides 
by the tough margin 
of the mantle in 
which they are 
embedded. In most 
cases the valves arc 
close together, extend 
right across the back, 
and are exposed; but 
in some genera they 
are far apart (Cryp- 
toplax), and in others 
entirely covered by the mantle ( Cryptochiton ). The foot occupies the entire 
ventral surface of the body, and the mantle covers the upper surface, extend¬ 
ing laterally beyond the shell. This portion is known as the girdle, and is 
nearly always covered with spines, scales, or spicules, which, to some extent, are 
common chiton ( Chiton squamosus ). 
