232 SHELLS OF MOLLUSCA. PART in. 



where they undergo a kind of incubation. At a certain 

 time the yellow yolk of the egg is divided into a granu- 

 lar mass, which separates from the liquid albumen and 

 produces cilia. The cilia cause the albumen to revolve 

 round the interior of the egg ; at last the granular mass 

 revolves with it, while at the same time it rotates about 

 its axis in a contrary direction at the rate of six or 

 eight times in a minute. When still in the egg, all the 

 organs of the little embryos are formed in succession, 

 even the little valves of the shells are seen to open and 

 shut, but the embryos are hatched before they leave 

 the parent, and swim about in the cavity of the external 

 gill. 



Shells of the Mollusca. 



When these mollusks come into the water, they soon 

 find their transparent white shell too small, and begin 

 to increase its size by means of the mantle, which is 

 an exquisitely sensible fleshy envelope applied to the 

 back of the animal, extending round its sides like 

 a cloak, only meeting in front, and it is for the most 

 part in close contact with the whole interior of the 

 shell. Its edges are fringed with rows of slender ten- 

 tacles, and studded with glands, which secrete the 

 colours afterwards seen in the shell ; the glands in the 

 rest of the mantle secrete only colourless matter. 



When the animal begins to enlarge its shell, it at- 

 taches the borders of the mantle to the margin of the 

 valves, secretes a film of animal matter^ and lines it 

 with a layer of mucus containing carbonate of lime 

 and colour in a soft state, which soon becomes hard, 

 and is then coated internally by the other glands of the 

 mantle with colourless carbonate of lime. 



The two strata thus formed, one richly coloured, the 

 other white, often nacreous and brilliantly iridescent, 



