SPINES AND EXTERNAL APPENDAGES. 



521 



abode, all that portion of its body which was covered by the shell is 

 seen to be invested with a thin mantle (fig. 261, a) precisely analogous 

 to that of the Limpet : from this pallial membrane the nacreous lining of 

 the shell exudes. But around the aperture the mantle swells into a 

 thick glandular collar (6), correspondent in function with the margin of 

 the mantle in Patella, and in like manner provided with glands adapted 

 to furnish colouring matter. From the collar, therefore, those layers 

 are secreted by which the extension of the shell is accomplished ; and 

 as the deposit is in this case far more abundant in one direction than in 

 another, the shell, as it expands, assumes more or less completely a 

 spiral shape. Wherever glands for secreting coloured pigment exist, 

 corresponding bands or coloured patches are produced as the layers of 

 growth are formed, and the exterior of the shell is thus painted with 

 the tints peculiar to the species. 



(1372.) In many marine Gasteropods, spines and various external 

 processes are found projecting from the outer surface of the shell, the 

 production of which depends upon the shape of the margin of the mantle. 

 Let the reader imagine one of these ornamented shells to be trans- 

 parent, so as to permit the contained animal to be delineated in situ, as 

 in the annexed sketch of Pterocera (fig. 262) ; and the collar, which 



Fig. 262. 



Animal of Pterocera in situ. 



forms the layers of growth, will be found to exhibit fringes or processes 

 precisely resembling those upon the shell itself. But it is only at 

 intervals that, as the growth of the mollusk proceeds, these pallial 

 appendages encase themselves in a calcareous covering, every such in- 

 terval being distinctly indicated upon the exterior of the shell by the 

 spaces between the successive rows of spinous projections that mark 

 the terminations of so many distinct periods in its formation ; so that 

 the number of ridges or rows of spines is, of course, correspondent with 

 the age of the creature within. 



