168 Conchology. 



light is very differently reflected. But there is another cause for 

 this difference of color in these circumstances. When a large 

 piece of shell is removed, the first layer which is formed is usual- 

 ly white. The particles of the fluid which are necessary for the 

 formation of the shell of this color, seem to be more easily excret- 

 ed from the surface of the body, than the particles of fluid which 

 go to the formation of any other color. It is observed that the 

 body of the animal is covered with this fluid long before there is 

 any appearance of secretion about the neck. This liquid is ex- 

 tended to the neck, and this produces a new layer of white shell; 

 but as this layer is extremely thin and transparent, it does not 

 prevent the usual secretion of the coloring matter at the neck to 

 appear. In this period of the process if the animal retire within 

 its shell, the new layer, still adhering in many points to its body, 

 and not having acquired sufficient solidity, will be distorted and 

 wrinkled; and not only exhibit that inequality of surface which 

 generally appears in shells thus formed, but this arrangement of 

 stripes or colors will also be destroyed. 



It would be a very false conclusion from this account of the 

 mode of the formation of the stripes which appear on certain spe- 

 cies of shells, that the external surface of all shells should be 

 marked with colors, or should be uniformly of the same color; 

 and that there should be no shells whose external surface is mark- 

 ed with different spots, differently arranged, of an irregular figure, 

 and separated from each other by unequal intervals. For if it has 

 been shown that these colors are produced on the surface of the 

 shell, only by means of the secretory organs, situated on the neck 

 of the animal, it cannot be supposed that the same effects will 

 follow, unless the animal is placed in the same circumstances. 

 These secretory organs, therefore, must exist during the entire 

 formation of the shell, to furnish the same quantity of coloring 

 matter during the whole of its progress. But if it happen, on the 

 contrary, that these organs undergo any change; if the pores 

 through which the liquid is poured to form a shell or part of a 

 shell of a brown color, become too large or too small, or in other 

 respects change their form, after having poured out a certain 

 quantity of this fluid; and that those which furnish the fluid of 

 which the white part of the shell is composed, are also changed, it 

 must happen that the shell which is produced is marked with dif- 

 ferent black and white spots, combined with a degree of irregular- 

 ity corresponding to the change on the secretory organs. This 

 will appear to be the case, by attending to the changes which 

 take place in the secretory organs of snails which produce color- 

 ed shells; for in them it may be observed, that the colors aredis- 



