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 Cnjptoplax 

 ( - 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 (Cryptopkix and 

 some species of Acanthochitori) 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 : (a) 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, 

 to which it is firmly attached. These pro- 

 jections of the outer or peripheral margins 



FIG. 23. 



Young Toniciafastigiata, 

 Gray, dorsal aspect, show- 

 ing the first shell-eyes. I, 

 first shell -plate; II, the . 



first eyes on the second of the valves are termed "insertion plates ; 



shell -plate; III, third ,-, n v , , , j , , 



sheii-piate. they are generally slit or notched to lorm 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 

 "girdle," as it is called, is covered with chitinous or calcareous 

 spicules of various shape, acicular or squamose. Each 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 



