CHAP. XIV 



POLYPLACOPHORA 



401 



The Pohjiylaeophora are characterised, externally^ by their 

 usually articulated shell of eight plates or valves^ which is 

 surrounded and partly kept in position 

 by a muscular girdle. These plates over- 

 lap like tiles on a roof in such a way that 

 the posterior edge of the first, cephalic, 

 or anterior valve projects over the an- 

 terior edge of the succeeding valve, which 

 in its turn overlaps the next, and so on 



throughout. Seven- 



valved monstrosities 



very rarely occur. 

 A certain portion 



of each valve is cov- 

 ered either by the 



girdle or by the valve 



next anterior to it. 



This portion, which is 



whitish in colour and 



non-porous in struct- 



i: 



t 



separated to show the 

 various parts (anterior 

 valve uppermost) : a, a, 

 articulamentum ; &, beak ; 

 j, jugum; pi, pi, pleura; 

 t, t, tegmentum. 



Ure, forms part of an Fig.261.— Valves of a C/ii^on 



inner layer which 

 underlies the rest of 

 the substance of the 

 valve, and is called 

 the articulamentum. 



The external portion of the valves, or teg- 

 me7itum^ is generally more or less sculptured, 

 and is largely composed of chitin, impregnated 

 with salts of lime, thus answering more to a 

 cuticle than to a shell proper. It is very 

 porous, being pierced by a quantity of minute 

 holes of two sizes, known as megalopores and 

 micropores., which are connected together by 

 Fig. 262. —Valves of minute canals containing what is probably 

 CAi^one //MS separated fi]3Pous or nerve tissue, the mouths of the pores 



out (anterior valve , . ^ 



uppermost) : a, a, ar- being occupied by scusc organs connected 

 ticuiamentum ; t, t, ^y\^\-y these nerves. The tegmentum of the 



tegmentum, x 2. . . . . ° 



six intermediate valves is generally divided 

 into three triangular areas by two more or less prominent ribs, 



VOL. Ill 2D 



