506 ZOOLOGY. 



so that the shell as it grows in thickness also enlarges rapidly. 

 In general, the distinction of laminae whilst forming is 

 less marked, and the new matters are often deposited only on 

 the edge of the shell, and in such a way that their molecules 

 correspond exactly to the molecules of the part already con- 

 solidated ; this gives to the whole a fibrous structure. Colours 

 the most varied and most agreeably disposed ornament these 

 shells, and often vary with age. Very generally they are 

 altogether superficial, and seem to depend on a sort of dyeing 

 process produced by the skin of the animal, which is painted 

 in a manner corresponding to its envelope. The colouring 

 matter appears to be deposited on the shell at the moment of 

 its formation ; it is also more lively as the shell is younger ; 

 it is produced by the edge of the mantle. In fact, if a shell 

 be broken, and the animal happens to repair this accident, the 

 part newly formed is always white where it has not been in 

 contact with the edge of the mantle ; and if it corresponds 

 with this edge, it assumes the colour which this presents at 

 the point of contact. Thus when the edge is spotted, corre- 

 sponding spots are found upon the edge of the shell ; and as 

 this elongates, these spots become confounded with those 

 already formed, and produce lines perpendicular to the striae 

 of increase, or do not unite with them, and remain insulated, 

 according as the mantle remains immovable, preserving with 

 the margin of the shell the same relations.; or, on the other 

 hand, that by the movements of the animal it often changes 

 its position. Sometimes the secretion of colouring matter also 

 varies with age, and accidental circumstances may equally 

 modify it. Light, for example, exercises over the phenome- 

 non a very remarkable influence, and not only the shells which 

 are the most exposed to the action of this physical agent are 

 generally the most brilliantly coloured,^but when the mollusc 

 lives fixed on a rock, or partly concealed under a sponge or 

 some other opaque body, the portion of the shell thus placed 

 in obscurity is always paler and duller than that exposed to 

 the contact of the sun's rays. 



595. The digestive apparatus of these animals is strongly 

 developed. There exists always a large liver, and often we 

 find salivary glands and organs of mastication ; but the intes- 

 tines are never supported by a mesentery. Their blood is 

 colourless, or slightly bluish, and circulates in a very complex 

 apparatus, composed partly of arteries and of veins, and partly 

 only of lacunaB. A heart, formed of a ventricle, and of one or 



