CLASS II. GASTEROPODA: ORDER 1. PULMONIFERA. 505 
Class II. €fASTEirOPOI>A. 
This term, derived from the Greek gaster, the belly, and po?is, foot, signifies helly-ioalJcers. 
The organ by which they move, 
as is shown in the common snail, 
consists of a broad, muscular, 
disk-like foot, attached to the 
ventral surface, upon which the 
animal creeps very slowly with 
a gliding motion. The muscu- 
lar movements may be seen fol- 
lowing ea<;h other in rapid waves, 
when a snail is climbing a pane 
of glass. There are numerous 
species, which greatly vary in 
form. All, however, have a dis- 
tinct head ; respiration is effected 
by branchiae, or a pulmonary sac ; 
the organs of the senses are ten- 
tacles of various forms ; the eyes 
are usually placed at the ends of 
tentacles situated upon the head. 
Jfo special organs of taste or smell have been detected, but there is good reason, from the discri- 
mination these animals show in the selection of their food, for believing that they possess them. 
The general form of the body is characteristic of the class ; from the preponderance of one side 
of the body, the whole, during growth, acquires a spiral form ; it is only in some naked species 
that we find the body symmetrical. Most of the Gasteropoda close the aperture of their shell 
by a horny or calcareous plate, called the 0})erculum. Most of the species are oviparous ; a few 
are ovo-viviparous. The sexes are generally separate, but many are hermaphrodites. The 
young are alwaj^s provided with a shell while in the egg. These animals are divided into three 
orders, the Pulmonifera^ Branchifera^ and Heteropoda. 
ORDER 1. PULMONIFERA. 
This term, from the Latin, pulmo, a lung, and fero, to bear, refers to the fact that the animals 
of this order breathe air by means of lungs, and not water by means of branchiae. It includes 
several species popularly known under the name of Snails and Slugs. 
THE HELICIDiE. 
Genus HELIX : Helix. — To this belongs the Common Gakden-Snail, H. aspera. This is 
furnished with four tentacula, two of Avhich are smaller than the others ; at the end of these, 
which the animal pushes out or draws back like telescopes, are blackish knobs, which are the 
eyes. It lays eggs about the size of peas, which are of a soft transparent substance. By closely 
examining with a magnifying lens, the young snail may be seen in the egg, with its embryo shell 
on its back. The snail is extremely tenacious of life, in evidence of which numerous examples 
have been cited ; among them is the following, which is furnished by Mrs. Loudon : a Mr. S. 
Simon, a merchant of Dublin, whose father, a Fellow of the Royal Society, and a lover of natural 
history, left him a small collection of fossils and other curiosities, had, among them, the shells 
of some snails. About fifteen years after his father's death, he gave to his son, a child of ten 
years old, some of these snail-shells to play with. The boy placed them in a flower-pot, which 
Vol. II.— 64 
