Conchology. 169 



tinct and well marked in some, towards the opening, while they 

 are scarcely perceptible on the first turn of the spire towards the 

 tip of the shell; and these changes of color cannot be supposed to 

 exist without a corresponding change on the secretory organs. 



The fluidity of the liquid for the formation of the shell has 

 probably also some effect in the regular distribution of the colors 

 which appear in some species. It is easy to imagine that some 

 animals may secrete a fluid for the formation of the shell, of such 

 a degree of fluidity as to flow easily from one place to another, 

 and thus produce irregular marks on the shell. But besides, if 

 there are secretory organs situated on the neck of the animal, 

 which prepare fluids of different colors; if the animal moves, or 

 is disturbed by any means, when these fluids are excreted on the 

 surface, the colors will appear in a different place from their origin- 

 al distribution, or to be mixed and blended together, and thus 

 occasion that irregularity which is observed in those parts of 

 shells which have been last produced, or renewed. 



But it will be necessary to have recourse to the first of thes6 

 causes, namely, to the change of structure in the secretory organs 

 of the neck, to explain the regular distribution of the round spots, 

 or of those of a square or rectangular figure, with which certain 

 shells are marked, and to suppose that those vessels which are ar- 

 ranged in a square or rectangular manner, which furnish peculiar 

 fluids, are shut or open at different periods. It may happen that 

 the developement of a great part of the animal, occasioned by a 

 more vigorous growth in certain species than in others, may, in 

 some cases, be the only cause of those regular spots, sometimes 

 white on a colored ground, and sometimes colored on a white 

 ground, which the shell exhibits, if the glands which secrete the 

 coloring matter correspond in their distribution, to that of the di- 

 visions on the shell, and if they occupy a greater space on the 

 neck than is usual in other species. In this way may be account- 

 ed for, the regularity of those marks, and the increase of their 

 size, which is usually proportioned to that of the turns of the 

 spire, from the consideration of the secretory organs of the ani- 

 mal enlarging in the same proportion as the other parts of its 

 body; and their effects in the formation of the shell correspond- 

 ing to the developement of those parts. Hence it follows, that 

 the largest marks are observed on the external convolutions of the 

 shell. 



According to Reaumur, the last layer of the shell which is form- 

 ed from a fluid secreted from that part of the surface of the ani 

 mal's body which does not reach the neck, should be white, and 

 this is most generally the case. In those shells which are inler- 

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