220 



THE MECHANICAL FUNCTIONS. 



which several shells exhibit, and which have arisen 

 periodically during their growth from their outer 

 surface. In the Murex these processes are often 

 exceedingly numerous, and occur at regnlar inter- 

 vals, frequently shooting out into various anomalous 

 forms. In many shells of the genus Stromhus the 

 spines are of great length, and are arranged round 

 the circumference of the base, being at first tubu- 

 lar, and afterwards solid, according to the period 

 of growth. This is exemplified in the Pterocera 

 Scorpio (Lamarck), of which Fig. 110 shows the 

 early, and Fig. Ill the later period of growth. 



A limit has been assigned by nature to the growth 

 of molluscous animals, and to the shells which they 

 form ; and there is a certain epoch of their exist- 

 ence, when considerable changes take place in the 

 disposition of the mantle, and in its powers of secre- 

 tion. Often we find it suddenly expanding into a 

 broad surface, and adding to the shell what may 

 be termed a large lip. Sometimes no sooner has 

 this been accomplished than the same part again 

 shrinks, and the mantle retires a little way within 

 the shell, still continuing to deposit calcareous 

 layers, which give greater thickness to the adja- 



