82 



Invertebmta. 



pearl 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 vertical, 

 prisms, while the inner surface consists of fine, minutely 

 corrugated layers, which are often finely waved, and so 



FIG. 



Diagram of the anatomy of a La melli branch, or Bivalve Mollusc. -, stomach; 

 /, intestine surrounded by the liver. The two tubes on the left marked 

 by arrows are the canals of the siphon. The current of water enters by 

 the lower and sets out by the upper tube, a, the anus ; b, hinder abduc- 

 tor muscle under which is the parieto-splanchnic nerve ganglion ; c, 

 heart ; d, nerve ganglia ; e, fore adductor muscle ; f, mouth with labial 

 tantacles, usually only two in number ; the dotted line from g- passes 

 between the pedal ganglion which lies in front of it and the ear- vesicle ; 

 /z, gills. The right valve of the shell has been removed, and the right 

 side of the mantle. 



decompose the rays of light which fall on them, thus pro- 

 ducing 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 constantly 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 



