438 MOLLUSCA SUB-KINGDOM VI 
breathing pore. The sides of this pore, in operculated snails, are often pro- 
duced outwards, so as to form a closed or cleft tube, corresponding with which 
there is frequently a canal-like process of the shell. 
Gastropods are remarkable for their extreme differentiation of the repro- 
ductive organs. The sexes are distinct in the Prosobranchia and Heteropoda, 
but united-in the Opisthobranchiata, Pteropoda, and Pulmonata. The ovarian 
and seminal ducts of hermaphrodites sometimes open into a common cloaca, 
or they may terminate in separate openings. 
The shell, as has already been remarked, is secreted by the mantle, and is 
limited in form and size by the configuration of the intestinal sac. It is 
composed of a chitinous substance (conchiolin) infiltrated with lime carbonate, 
or exceptionally with sulphate of lime in small quantities. Shell characters 
are of great importance in distinguishing genera and species, but their value 
in classifying larger groups is comparatively slight, owing to the fact that 
very similar shells are often developed among forms which differ widely in 
their general organisation. Two forms of shell-habit occur, the symmetrical 
and the spiral. The first are flat-, conical, or saucer-shaped, and characterise 
only a few groups (Cyclobranchia, Aspidobranchia, Pulmonata). Transition 
forms between the symmetrical and spiral are to be observed in conical shells 
with slightly inrolled beaks. Exceptional forms of the spiral shell are seen 
in Vermetus, which is irregularly coiled, and in Planorbis, Bellerophon, and 
Atlanta, coiled in one plane (discoidal). Usually the shell forms a screw-like 
spiral, and rests upon the back of the creature in such a way that the apex is 
directed upward and backward, the aperture forwar d and downward. Holding 
the shell upright so that the apex is above, and the aperture below, facing 
the observer, it is said to be right-handed or dextral when the opening is on 
the right side, and left-handed or sinistral when on the left side. By far the 
larger number of Gastropods are dextral ; but a few (Clausilia, Physa, Spirialis) 
are normally sinistral. Right-handed individuals of normally left-handed 
genera, as well as pathologic individuals of normally right-handed forms, are 
occasionally met with. 
In drawing and describing Gastropod shells, the apex is ordinarily directed 
upward, so that the right- or left-handedness may be seen at a glance. It is 
also customary to employ the terms above and below in the same sense as 
posterior and anterior. The height or length of the shell is measured by a 
line drawn from the apex to the lower margin of the aperture. 
The shell is to be considered as a more or less rapidly widening cone, 
which is wound either around an axial pillar, called the columella, or about a 
central tubular cavity. Each coil of the tube is termed a whorl, and all the 
whorls except the last one form together the spire. The last or body whorl 
is often very much larger than the preceding ; its lower, sometimes flattened 
surface is called the base. As a rule, the whorls are in contact with each 
other, each in succession either partly or entirely covering the preceding ; but 
in rare cases they form a loose spiral, in which the whorls are separated from 
one another. The spire is said to be convolute when the later whorls 
entirely conceal the earlier ones, as in Cypraeca. The line between two con- 
tiguous whorls is known as the suture. According to the manner of inrolling, 
various shell contours are produced, requiring numerous descriptive names, 
such as conical, auriform, turbinate, fusiform, cylindrical, spherical, oval, 
pyramidal, ete. 
