CHAPTER XL 



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 (Ohiton squamosus). 



