CHAPTER XI 



MOLLUSKS concluded 



CHITONS, TOOTHSHELLS, AND BIVALVES CLASSES Amphineura, 

 Scaphopoda, and Pelecypoda 



THE CHITON GROUP Class Amphineura 



THE mollusks 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 characterizes the external parts also 

 extends to the internal organization. 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 recognized 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 are 

 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 {Cryptochitori) . The foot occupies the entire 

 ventral surface of the body, and the mantle covers the upper surface, extending 

 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 

 (3368) 



's^rT-^ , ' ~^ 



COMMON CHITON (Chiton squamosus). 



