86 MAIfUAf OP THE MOLLUSCA. 



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 neritidoe 

 and auricuUdce dissolve all the internal spiral column * of their 

 shells ; the cone (Fig. 24, B) removes all but a paper-like 

 portion of its inner whorls ; 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. 93), until, in some cases, all resemblance to the young 

 shell is lost in the adult. 



The power which molluscs possess of dissolving portions of 

 their own shells is also exhibited by the murices in removing 

 those spines from their whorls which interfere with their growth ; 

 and by the purjpurce and others in wearing away the wall of 

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

 chemical. Some support is given to this view by the composi- 

 tion of the saliva of dolium galea (closely allied to the purpurce), 

 which has been exajnined by Professor Troschel. A chemical 

 analysis showed that it contained a minute proportion only of 

 organic matter, and consisted of 94 per cent, of water, the 

 remainder being almost entirely muriatic and sulphuric acids, 

 and the sulphates of magnesia, potash, and soda. The secretion 

 is apparently not used to assist digestion, since minute cal- 

 careous shells were found in the stomach uninjured. It is not 

 used for perforating stones ; and what its function is remains 

 uncertain. We mention these facts here to show that dilute 

 acids are secreted, which in some cases may be used for dis- 

 solving away the shell. The saliva, however, has no ejffect on 

 the inside of the shell of the dolium; indeed, it is said to be 

 unalterable by strong acids. {Monatsherichte der Academie in 

 Berlin, 1854, p. 486). 



Decollated shells. It frequently happens that as spiral shells 

 become adult they cease to occupy the uj)per 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 the shell truncated or decollated. This 

 happens constantly with the truncatellce, cylindrellce, and hidimus 

 decollatus; amongst the fresh- water shells it depends upon 

 local circumstances, but is very common with pirena and 

 cerithidea. 



Forms of shells. These will be described particularly under 

 * Thia is sometimes done by the hermit-crab to the shell it occujpies. 



