44 



OCEAN LIFE. 



a retrogressive movement. The simple apparatus by means of 

 which shells are constructed, is the external membranous layer 

 that invests the body of the Mollusc, the mantle, as it has been 

 termed, and, whatever the form of the shell, it owes its origin en- 

 tirely to this delicate organ. When the animal is engaged in- 

 increasing the dimensions of its abode, the margin of the mantle 

 is protruded, and firmly adherent all round to the circumference 

 of the valve with which it corresponds. Thus circumstanced, it 

 secretes calcareous matter, and deposits it in a soft state upon 

 the extreme edge of the shell, where the secretion hardens and 

 becomes converted into a layer of solid testaceous substance. 

 At intervals this process is repeated, and every newly-formed 

 layer enlarges the diameter of the valve. The concentrix strata 

 thus deposited remain distinguishable externally, and thus the 

 lines of growth marking the progressive increase of size may 

 easily be traced. 



" It appears that at certain times the deposition of calcareous 

 substance from the fringed circumference of the mantle is much 

 more abundant than at others : in this case ridges are formed at 

 distinct intervals ; or, if the border of the mantle at such periods 

 shoots out beyond its usual position, broad plates of shell, or 

 spines of different lengths, are secreted, which, remaining per- 

 manent, indicate, by the interspaces separating successively de- 

 posited growths of this description, the periodical stimulus to 

 increased action that caused their formation. 



u Whatever thickness this shell may subsequently attain, the 

 external surface is thus exclusively composed of layers deposited 

 in succession by the margin of the mantle ; and, seeing that this 

 is the case, nothing is more easy than to understand how the 

 colors seen upon the exterior of the shell are deposited, and as- 

 sume that definite arrangement characteristic of the species. 

 The border of the mantle contains in its substance colored spots; 

 these when minutely examined, are found to be of a glandular 

 character, and to owe their peculiar colors to a pigment secreted by 

 themselves ; the pigment so furnished, being therefore mixed up 

 with the calcareous matter at the time of its deposition, colored 

 lines are formed upon the exterior of the shell wherever these 

 glandular organs exist. If the deposition of color from the 



