16 COLOURS OF SHELLa 



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

 always wliite, if it has Bot been in contact with the edge of the 

 mantle ; but if it correspond to this edge^ it then assunies 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 stride, 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 acciden^tal circum» 

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

 remarkable influence on this phenomenon, for not only arc shells 

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

 narily most brightly edoured, but, when a niK>llusk lives fixed to 

 a rock, or in some spot hidden besieath 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 the 

 solar rays. 



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

 which inhabit them, are circumstances not yet determined. WhcD 

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

 ture and moisture for some time, shells usually change theif 

 colours and gradually become white. The antraal luatter which 

 enters into its composition is destroyed, and disappears little by 

 little ; the laminse separate from each other, particu'larly tinder 

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

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

 ©f 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 orgaasof mas- 

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

 assistance of a mesentery. 



12. The blood of these anira'als rs 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- 

 luska .' 



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



