BY A. J. JUKES-BROWNE. 
67 
that when the shell is thick this hole is not cut straight 
through, but narrows downward as if it had been made by 
a conical file, such as that used to sink the head of a screw. 
The reason of this is that the animal has been killed by one 
of his carnivorous fellows who has bored a hole in the shell 
by means of his dental ribbon, which is curved from side 
to side so that the central teeth come into action first and 
the lateral teeth are only brought to bear as the hole is 
deepened. 
Passing now to a consideration of the shell, we may first 
ask, what is the use of it? The primary reason for its 
existence seems to be the protection of the soft parts of the 
animal, and especially of the delicate gill or breathing 
organ ; it also serves as a basis of attachment for the 
muscles of the foot and of those by which the animal 
retracts itself within its shell. The scar of this attach¬ 
ment is clearly seen inside a limpet-shell as a mark in the 
shape of a horseshoe, but in spiral shells the retractor 
muscle is attached to the central axis or “columella” 
round which the shell is coiled. 
Here, also, it should be mentioned that most of the 
spiral Gasteropoda have a horny or shelly plate attached 
to the hinder part of the foot, so arranged that when the 
animal withdraws itself this plate or “ operculum ” comes 
last and closes the aperture of the shell, so as to afford pro¬ 
tection from attack in that quarter. The shape and sub¬ 
stance of the operculum vary in different genera, and 
thus it becomes of assistance in classification. It is 
generally fully developed and very tight-fitting in the 
spiral vegetarians, while among the carnivorous genera, 
though many possess opercula, they are often too small to 
close the aperture completely, and in some cases they are 
absent altogether. 
The shell itself is either conical or spiral, and the spiral 
form may be regarded as a twisted cone. The Limpet has 
the simplest form of shell; the Bonnet-shells (Capulus and 
Crucibulum) show how the apex of the cone becomes twisted 
into the beginning of a spire. The Ear-shells, again 
(Haliotis),- show how this can be coiled into a flattened 
whorl, and from this the transition is easy to the more 
elongate spiral form of a turbinate shell such as the 
Winkle ( Littorina ) or the Top-shells (Turbo and Trochus ). 
Some Gasteropods have a tube or siphon, through which 
water is passed into the gill-chamber, and when such a 
siphon exists, the aperture of the shell is notched at the 
f 2 
