INTRODUCTORY CHAPTER. 13 



be spiral, cup-shaped, or otherwise, according 

 to circumstances. This can be best explained 

 by a few references : thus, the simplest form 

 of shell is that of the Limpet, which has a very 

 small portion (represented at A in the figure), 



A 



formed at the period of its birth ; as it increases 

 in size, and finds this inadequate to protect its 

 body, it expands the border of its mantle, and, 

 by the process already described, enlarges its 

 shell all round, until it presents the appearance 

 of A B ; in like manner, at different periods, as 

 its growth requires, it enlarges it to C, D, and E, 

 at which time, having attained its full size, it 

 ceases to require any farther enlargement, and 

 the shell is then completed or mature. 



The Limpet presents the most simple method 

 of growth, as in it the mantle is extended 

 equally at every part of the circumference of 



