GASTEROPODA. 



421 



and in like manner provided with glands adapted to furnish colour- 

 ing 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 com- 

 pletely a spiral shape. Wherever glands for secreting coloured 

 pigment exist, corresponding bands or coloured patches are pro- 

 duced as the layers of growth are formed, and the exterior of the 

 shell is thus painted with the tints peculiar to the species. 



(461.) 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 transparent, so as to permit the contained animal to be de- 

 lineated in situ, as in the annexed sketch of Pterocera (Jig. 196) ; 



Fig. 196. 



and the collar, which 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 interval 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. 



(462.) Several of the Pectinibranchiate genera are provided with 



