MOLLUSCA. 261 
plate (operculum) is secreted on the foot, which closes the 
aperture when the animal withdraws into its shell. In 
locomotion, the shell is carried with the apex directed 
backward. 
The body of most Gasteropods is unsymmetrical, the 
organs not being in pairs, but single, and on one side, 
instead of central. The mantle is continuous round 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 situ- 
ated, in the majority, on the right side of the back. 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 sep- 
arate 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 common name, “ Sea-but- 
terflies.’ Many have a delicate, trans- 
parent shell. The head has six ap- 
pendages, armed with several hun- 
dred thousand microscopic suckers— 
a prehensile apparatus unequaled in yy. 990,—A Pteropod (Hy- 
complication. » Pteropods ‘occur: in 4 *identata). Atlantic. 
every latitude, but generally in mid-ocean, and in the 
arctic regions are the food of Whales and Sea-birds. 
2. Opisthobranchs.—These low Gasteropods are, for the 
