366 



FORM AND GROWTH OF SHELL. 



round a central line in a corkscrew-like mode, a shell is formed 

 like that of the common Snail, or the Pleurotoma (Fig. 582). 

 From forms of this kind, we may return to the Dentalium, 



which has a long straight cone, like 

 that of the Limpet narrowed and 

 drawn out, by the Scalaria or 

 Wentletrap, in which the coils of 

 the spire touch each other only by 

 their ribs ; and by the Magilus and 

 Vermetus (Fig. 583), in which the 

 commencement only of the shell possesses a spiral form, the 

 remainder being prolonged into a tube which nearly approaches 

 the straight direction. When the whorls revolve around a ver- 

 tical line, instead of remaining in the same plane, a sort of 

 central pillar is formed, which is termed the columella, (Fig. 

 558) ; this is usually grooved at its lower part, for the passage 

 of water. to the respiratory organs. 



909. The margin of the shell is not unfrequently fringed with 

 spines, as in the Murex ; these are formed (as are similar ap- 



FIG. 558. SECTION OF ACHATINA 

 COLUMNAUIS. 



FIG. 559. MUREX TENUISPINA. 



pendages in the Conchifera,) by prolongations of the Mantle ; 

 and the dissimilar number of them in different specimens has 

 caused the establishment of many species, which, now that the 

 habits of the animal are better known, prove to be but varying 

 forms of the same. For it has been ascertained that the animal 

 has not only the power of forming new spines, but of removing 



