INTRODUCTORY CHAPTER. 17 



manner are produced the prickles, ridges, and 

 other excrescences visible in many genera, the 

 Murex, for example. Analogous to these varia- 

 tions in the deposit of shell by the mantle, but 

 differing, inasmuch as they only occur at one 

 period, are the developements which take place 

 when the mollusc has arrived at its full growth. 

 In the cut are represented the immature and 



mature states of the same shell. It will be seen 

 that they differ widely ; and the variation has 

 been caused by the mantle forming the exten- 

 sion of shell with its accompanying projecting 

 tubes, as a completion to the animal's habita- 

 tion : it marks the termination of its labours, 

 for the shell having now become perfect, and 

 c 



