86 MANUAL OF TttK MOLLUSC A. 



Yet at this time they would scorn to require more fipacfe 

 and accommodation in their houses than before, and there are 

 several curious ways in which this is obtained. The ncritidce 

 and auriciiUdoi 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 externdlJy with its overlapping mantle 

 (Fig. 98), 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 j)urpurce 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 gcdea (closely allied to the purpurce), 

 which has been examined by Professor Troschcl. A chemical 

 analysis showed that it contained a minute pro2:)ortion only of 

 organic matter, and consisted of 94 i)cr cent, of water, the 

 remainder being almost entirely muriatic and sulphuric acids, 

 and the sul]")luites of magnesia, potash, and soda. The secretion 

 is ai")j)arcntly 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. AVc mention these fticts 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 effect on 

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

 unalterable by strong acids. {Monatsbericlite der Academie in 

 Berlin, 1854, p. 48G). 



Decollated shells. It frequently haj)pens that as spiral shells 

 become adult they cease to occupy the U2')per part of their 

 cavity; the space thus vacated is sometimes filled with solid 

 shell, as in magilus ; or it is i:)artitioned off, as in vei^mefuSy 

 ciiompJialus, turritella, and triton (Fig. G2). Tlie 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 truncatellcc, cylindrellce, and hidimus 

 decollatus ; amongst the fresh- water shells it depends upon 

 local circumstances, but is very common with pzVcjia and 

 ccrithidea. 



Forms of shells. These will be described particularly under 

 * This is swaetimcs done by the hermit-crab to tlic shell it occupies. 



