82 Invertebrata. 



lustre. The cause of the difference in appearance is 

 seen on making a microscopic section through a shell, 

 as the outer surface is composed of long, nearly ver- 

 tical, prisms, while the inner surface consists of fine 

 layers whose edges overlap each other. These edges 



Diagram of the anatomy of a Lamellibranch, or Bivalve Mollusc, g, stomach ; 

 i intestine surrounded by the liver, the two tubes on the left marked 

 by arrows are the canals of the siphon, a, the anus ; b, hinder adductor 

 muscle ; c, heart ; d, nerve ganglia ; e, fore adductor muscle f t mouth ; 

 h, gills. 



are often finely waved, and so decompose the rays of 

 light which fall on them, thus producing the iridescent 

 appearance seen in so many shells. The nacreous or 

 mother-of-pearl layers are secreted by the surface of 

 the mantle, while the prismatic material is formed by 

 the margin of that structure. Thus the shell is con- 

 stantly increasing in size by the formation of new 

 prismatic matter, the lines of growth being the con- 

 centric curves before noticed. The edge of the mantle 



