16 COLOURS. OF SHELLS. 



mal succeeds in repairing the accident, the part newly formed ia 

 always white, if it has not been in contact with the edge of the 

 mantle ; but if it correspond to this edge, it then assumes the 

 colour that the latter presents at the point touched. For exam- 

 ple, when this edge is spotted, we find corresponding spots on 

 the margin of the shell, and, in proportion as the latter is elon- 

 gated, these spots become confounded with those previously 

 formed, and produce lines perpendicular to crossing strite, or 

 they do not join the latter, but remain isolated, according as the 

 mantle remains unmoved, and preserves the same relation to the 

 margins of the shell, or 'frequently changes its position according 

 to the movements of the animal. Sometimes the secretion of 

 the colouring matter varies with age; and accidental circum- 

 stances may also modify it. Light, for example, exerts a very 

 remarkable influence on this phenomenon, for not only are shells 

 that are most exposed to the action of this physical agent, ordi- 

 narily most brightly coloured, but, when a mollusk lives fixed to 

 a rock, or in some spot hidden beneath a sponge, or other opaque 

 body, the part of the shell thus placed in darkness is always 



/ dull, and paler than that which is exposed to the contact of th 

 solar rays. 



The duration of shells, and the length of time the animals live 



i which inhabit them, are circumstances not yet determined. When 

 exposed to the action of the air and the vicissitudes of tempera- 

 ture and moisture for some time, shells usually change their 

 colours and gradually become white. The animal matter which 

 enters into its composition is destroyed, and disappears little by 

 little ; the laminre separate from each other, particularly under 

 the alternate influence of heat and cold, and become finally re- 

 duced to a calcareous powder, which is washed away by currents 

 of water. 



11. All mollusks are provided with an alimentary canal, which 

 is more or less folded on itself and open at its two ends, either at 

 the extreme points of the body, or at points more or less near to 

 each other. There is always a voluminous liver, and we often 

 find them furnished also with salivary glands and organs of mas- 

 tication ; but the intestines are never held in their place by the 

 assistance of a mesentery. 



12. The blood of these animals is cold and colourless or slightly 

 bluish, and circulates in a very complicated vascular apparatus, 

 composed of arteries and veins. A heart, formed of a ventricle (v) 



11. What are the general characters of the digestive apparatus of mol- 

 lusks .' 



12. What is the nature of the blood of mollusks ? How is it circulated ? 



