44 MANUAL OF THE MOLLTJSCA. 



growth. But the most characteristic change is 

 the thickening and contraction of the aperture 

 in the univalves. The young cowry (fig. 27) 

 has a thin, sharp lip, which becomes curled in- 

 wards, and enormously thickened and toothed in 

 the adult ; the pteroceras (pi. 4, fig. 3) deve- 

 lopes its scorpion-like claws, only when full- 

 grown ; and the land-snails form a thickened lip, 

 or narrow their aperture with projecting pro- 

 cesses, so that it is a marvel how they pass in 

 and out, and how they can exclude their eggs, -^ 

 (e. g. pi. 12, fig. 4, anastoma; and fig. 5, helix 

 hirsuta). 



Yet at this time they would seem to require more space and 

 accommodation in their houses than before, and there are several 

 curious ways in which this is obtained. The neritida and auri- 

 culiddB dissolve all the internal spiral columnf of their sheljs ; 

 the cone (fig. 24, B,) removes all but a paper-like portion of its 

 inner whirls ; the cowry goes still further, and continues removing 

 the internal layers of its shell-wall, and depositing new layers 

 externally with its overlapping mantle (fig. 76), until, in some 

 cases, all resemblance to the young shell is lost in the adult. 



The power which mollusks possess of dissolving portions of 

 their own shells, is also exhibited by the murices, in removing 

 those spines from their whirls which interfere with their growth ; 

 and by the purpurce and others in wearing away the wall of their 

 aperture. The agency in these cases is supposed to be chemical. 



Decollated shells. It frequently happens that as spiral shells 

 become adult they cease to occupy the upper part of their cavity ; 

 the space thus vacated is sometimes filled with solid shell, as in 

 magilus ; or it is partitioned off, as in vermetus, euomphalus, 

 turritella and triton (fig. 62). The deserted apex is sometimes 

 very thin, and becoming dead and brittle, it breaks away, leaving 



* Cypraa testudinaria, L., young. 



f This is sometimes done by the hermit-crab to the shells it occupies. 



