50 ORGANS OF SUPPORT, 



jects, naked and acute, 

 from the opposite end of 

 the shell. But as it ap- 

 proaches maturity, (Fig. 

 23. 4,) the upper lip ex- 

 pands, becomes effuse, 

 extends to the right and 

 to the left side, so as to conceal the apex of the spire, (Fig. 

 23. 4. a,) and lengthen the canal (Fig. 23. 4. ,) and it 

 shoots upwards several processes which are at first thin 

 and hollow open canals, (Fig. 23. U c,) and are gradually 

 filled up with successive layers, and converted into solid 

 spines, which entirely change the appearance of the shell. 

 These changes of the upper lip take place periodically, 

 and at regular intervals during the development of many 

 shells, as in the murices, thus producing transverse rows 

 of variously formed processes from the outer surface of all 

 the turns of the cone. The young and the adult shells 

 of the cypraa are scarcely recognisable as belonging 

 to the same individual, from the changes of form they 

 experience at maturity. The form of the young shell of 

 the cypraa exanthema is represented in Fig. 23. 2, where 

 the aperture is wide, the upper lip thin and even, the 

 canal (b) projecting, and the apex (a) of the spire extended 

 and free. But in the adult form (Fig. 23. 1,) of the same 

 shell the aperture is contracted, narrow and serrated, the 

 upper lip is thickened and rounded backwards, the canal is 

 converted into a groove, and the whole of the spire is 

 covered and concealed. These adult changes in the cypraea 

 are produced by the extension of the sides of the mantle 

 over the upper and lower lips of the shell, and now 

 the new layers are added to the exterior surface, as if 

 it were an internal shell, and the line of junction of the 

 two enveloping folds of the mantle is marked by a trans- 

 verse discoloration or groove on the exterior of the shell 

 along the whole of its convex dorsal part. By this ad- 

 dition of new layers to its whole exterior surface, the 

 adult cyprsea has a smooth, glistening, naked and variously 

 coloured exterior, like the interior surface of most other 

 shells, and can present no rough epidemic covering when 

 arrived at that state. In the cypraea, the dolium, the 



