POLYPLACOPHORA. 



817 



area (fig. 732, b, m) and two lateral areas (/). The upper or 

 exposed surface of the valves is spoken of as the "tegmentum,"' 

 while the " articulamentum " is a peripheral zone which supports 

 and is partly covered by the 

 tegmentum. The shell is en- 

 tirely surrounded by, and more 

 or less extensively embedded 

 in, the "girdle" or "zona" 

 formed by the border of the 

 mantle, and variously covered 

 on its upper surface with 

 scales, spines, or bristles. 

 The cephalic and the six in- 

 termediate plates have their 

 apices at their posterior mar- 

 gin, but the apex of the anal 

 plate is usually anterior. 



The cephalic valve is usu- 

 ally semicircular in shape, and 

 its front edge is generally 

 prolonged into an " insertion- 

 plate," or forward extension 

 of the "articulamentum," by which it is embedded in the mantle. 

 The intermediate valves are transverse in shape as a rule, and 

 their front edges are furnished with forward extensions of the arti- 

 culamentum, which serve to unite the successive valves, and are 

 known as the "apophyses" or "sutural laminae" (fig. 732, c). Pos- 

 teriorly the intermediate plates usually possess " insertion-plates." 

 The anal valve is variable in form and sculpturing, as well as in the 

 position of its apex, but it has in front a pair of apophyses, separated 

 by a median sinus (b), and it possesses an insertion-plate posteriorly. 

 The insertion-plate of the anal valve may be similar in form to that 

 of the cephalic valve, or may be different from it, and by this char- 

 acter the Chitons are grouped as " regular " or " irregular." 



a, Cephalic valve, B, one of the in- 

 termediate valves, and c, anal valve of Chiton 

 jiiagnificus, viewed from the dorsal aspect : ;;/, 

 Central area of the valve ; /, One of the lateral 

 areas of the valve ; c c, Apophyses or sutural 

 laminae ; b b, Sinus and jugal area. (After Gray.) 



The tegmentum of the recent Chitons exhibits two sets of superficial 

 circular apertures, one set large (" megalopores "), the other set of much 

 smaller size (" micropores "). These apertures lead into corresponding 

 sets of tubes, large and small, which perforate the shell vertically, and 

 open below into a plexus of horizontal canals running in the space be- 

 tween the apposed surfaces of the tegmentum and articulamentum. These 

 horizontal canals open internally, and lodge processes derived from the 

 mantle, which send out ramifications into the canals connected with the 

 "megalopores" and "micropores." These ramifications finally expand 

 into differently-sized papilliform bodies ("megalaesthetes" and " micraes- 

 thetes "), which appear to be peculiar organs of touch. Further, as shown 

 by Moseley, a number of the "megalaesthetes" are in some cases spe- 



vol. 1. 3 F 



