MOLLUSCA. 
273 
When the columella is hollow (perforated), the end is 
called the u umbilicus.” When the whorls are coiled 
around the axis in the same plane, we have a discoidal 
shell, as the Planorbis. The mouth, or “aperture,” of 
the shell is “ entire ” in most vegetable-feeding Snails, and 
notched or produced into a canal for the siphons in the 
carnivorous species. The former are generally land and 
fresh-water forms, and the latter all marine. In some 
Gasteropods, as the River-snails and most Sea-snails, a 
horny or calcareous plate {operculum) is secreted on the 
foot, which closes the aperture when the animal with¬ 
draws into its shell. In locomotion, the shell is carried 
with the apex directed backward. 
The body of most Gasteropods is unsymmetrieal, the 
organs not being in pairs, but single, and on one side, 
instead of central. The mantle is continuous around the 
body, not bilobed, as in Lamellibranchs. A few, as the 
common Garden-snail, have a lung; but the vast majority 
breathe by gills. The head is more or less distinct, and 
provided with two tentacles, with auditory sacs at their 
bases; two eyes, which are often on stalks; and a strap¬ 
like tongue covered with minute teeth. The heart is sit¬ 
uated, in the majority, on the right side of the back, and 
has two cavities. The nervous ganglia are united into an 
oesophageal ring or collar (Figs. 45,154). All, except the 
Pteropods, move by means of a ventral disk or foot. 
Gasteropods are now the reigning Mollusks, comprising 
three fourths of all the living species, and are the types 
of the subkingdom. They have an extraordinary range 
in latitude, altitude, and depth. 
Omitting a few rare and aberrant forms, we may sepa¬ 
rate the class into the following orders: 
1. Pteropods .—These are small, marine, floating Mol¬ 
lusks, whose main organs of motion resemble a pair of 
wings or fins coming out of the neck, whence the com- 
18 
