POLYPLACOPHORA 



401 



i\ 



The Polyijlacophora 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, ceph- 

 alic, or anterior valve projects over the 

 anterior 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 covered 

 either by the girdle or 

 1)y the valve next an- 

 terior to it. This por- 

 tion, which is whitish 

 in colour and non- 

 porous in structure, 

 forms part of an inner 

 layer which underlies 

 the rest of tlie sub- 

 stance of the valve, 

 and is called the arti- 

 cidamentmn. The ex- 

 ternal portion of the valves, or tegmentiim, 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 Ijy a quantity of minute holes of two 

 sizes, known as megalopores and micropores, 

 which are connected together ])y minute 

 Fig. 262. — Valves of canals Containing what is prol)ably filjrous 

 C7nYonc//i/s separated or ncrvc tissue, the mouths of the pores being 



out (anterior valve • i i , i ■ , i , 1 



uppeniiost): «,«, ar- occupied by seusc organs connected with these 

 ticuiainentum ; t, t, nerves. The tegmentum of the six inter- 



tegmeutum. x 2. ,. ^ , . n i- • n 1 • , 



mediate valves is generally divided into three 

 triangular areas by two more or less prominent ribs, which 

 VOL. Ill 2 D 



Fig. 261.— Valves of a Chiton 

 separated to show the vari- 

 ous parts (anterior valve 

 uppermost) : a, a, articu- 

 lameutuiu ; b, beak ; j, 

 jugum ; pi, pi, pleura ; 

 t, t, tegmentum. 



