86 MANUAL OF 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 neritide 
and auriculide 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 remoying 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 mwrices in removing 
those spines from their whorls which interfere with their growth; 
and by the purpure 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 doliwm galea (closely allied to the purpure), 
which has been examined 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- 
solying away the shell. The saliva, however, has no effect on 
the inside of the shell of the doliwm; indeed, it is said to be 
unalterable by strong acids. (Monatsberichte der Academie in 
Berlin, 1854, p. 486). 
Decollated shells. It frequently happens that as spiral shells 
become adult they cease to occupy the upper part of their 
cayity; the space thus vacated is sometimes filled with solid 
shell, asin 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 truncatelle, cylindrelle, and bulimus 
decollatus; amongst the fresh-water shells it depends upon 
local circumstances, but 1s yery common with pirena and 
cerithidea. 
Ferms of shells. These will be described particularly under 
* This is sometimes done by the hermit-crab to the shell it occupies, 
