384 PHYLUM MOLLUSCA. 
side of the expanded animal, close to the anterior edge of 
the shell, there is a large aperture through which air passes 
into and out of the mantle cavity. Within the same 
aperture is the terminal opening of the ureter. The food 
canal ends slightly below and to the right of the pulmonary 
aperture. All the three openings are close together. The 
anterior termination of ureter and food canal is one of the 
results of the twisting of the visceral mass forwards to the 
right. But still farther forward, at the end of a slight groove 
which runs along the right side of the neck, indeed quite 
close to the mouth, is the genital aperture. Lastly, an 
opening just beneath the mouth leads into the large mucus 
gland of the foot. 
Shell.—The right-handed spiral shell is a cuticular product made 
and periodically enlarged by the collar. Chemically it consists of 
carbonate of lime and an organic basis (conchin). The outermost 
layer is coloured, without lime, and easily rubbed off; the median 
layer is thickest, and looks like porcelain; the innermost layer is 
pearly. The twisted cavity of the shell is continuous, and the viscera 
extend to the uppermost and oldest part, 
As the shell is made, the inner walls of the coils form a central 
pillar (columella), as’ on a staircase, to which the animal is bound by 
a strong (columellar) muscle. Many Gasteropods bear on the foot a 
lid or operculum, of conchin or of lime, which closes the mouth of the 
shell. In Ae/¢x there is none; the ‘‘epiphragm” with which the 
shell is sealed in winter consists of hardened mucus, plus phosphate 
and a smaller quantity of carbonate of lime. It is formed very quickly 
from the collar region when cold weather sets in, has no organic 
connection with the animal, such as binds the operculum to the foot of 
the whelk, and is loosened off in the mildness of spring. 
Sinistral shells, with left-handed spiral, occasionally occur as 
variations. The shell, held with its summit towards the observer, has 
its aperture to the left. The internal organs are inverted, and at the 
start there is a reversal of the cleavage planes of the egg. 
Appearance after the shell is removed.—If the shell is 
removed carefully, so that nothing is broken except the 
columellar muscle, many structures can be seen without 
any dissection. The skin of the head and foot should 
be contrasted—(a) with the thick collar of the mantle; 
() with the mantle itself, which forms the loose roof of 
the pulmonary chamber ; (c) with the exceedingly delicate, 
much-stretched, and always protected skin of the visceral 
hump. The mantle is a downgrowth of the skin of this 
dorsal region. It is peculiar in the snail, in that its margin 
