178 



THE MECHANICAL TUNCTIONS. 



gradually filled up by these deposites; the process of the 

 mantle retiring to make way for their advance towards the 

 axis of the tube. In the course of time, every part of the 

 cavity is obliterated, the process of the shell becoming en- 

 tirely solid. Such is the origin of the many curious pro- 

 jecting cones or spines which several shells exhibit, and 

 which have arisen periodically during their growth from 

 their outer surface. In the Murex these processes are of- 

 ten exceedingly numerous, and occur at regular intervals, 

 frequently shooting out into various anomalous forms. In 

 many shells of the genus Strombus these spines are of great 

 length, and are arranged round the circumference of the 

 base, being at first tubular, and afterwards solid, according 

 to the period of growth. This is exemplified in the Piero- 

 cera 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 epocli of their existence, when consider- 

 able changes take place in the disposition of the mantle, and 

 in its powers of secretion. Often w^e find it suddenly ex- 

 panding into a broad surface, 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 deposite calcareous layers, which give greater thickness 

 to the adjacent part of the shell: and at the same time nar- 



