ON GASTEROPODA. oon 
generation are extremely complicated. Each individual has 
an ovary, two oviducts and a bladder; a testicle, an epi- 
dydimis, a deferential canal, and an excitatory organ. 
The nervous system of the helices is very considerable. 
There is a central part above the intestinal canal, composed of 
two very thick ganglia. The nerves which it furnishes from 
all its external edge are very numerous and very considerable. 
There is a peculiar ganglion for the generative apparatus, 
which receives a thick thread of communication from the 
cerebral ganglion, and furnishes threads to the different parts 
of the apparatus. 
The helices are found, as it would appear, in all parts of 
the earth. They are known in Europe, in Africa, in the two 
Americas, in Asia, and in Australasia. It is generally in 
humid situations that they are principally to be found; but 
they are also to be met with in warm and dry places, which is 
never the case with the limaces or slugs. They usually with- 
draw into the excavations of old walls, of rocks, under the 
bark of old trees, and even under ground. ‘They sink more 
deeply during the winter season, at least in our part of the 
world; for in climates where vegetation is continual, it is 
probable that the snails do not hybernate, or if they do retire, 
it is during the intense heat, and especially at- the season 
when no rain is falling. Before they enter into the torpid 
state, the helices of our climates withdraw their body entirely 
into the shell, and produce at its entrance a sort of momen- 
taneous opercle, or lid, to which the name of epiphragma 
has been given. It is evidently composed of calcareous 
molecules, not very abundant, united by an animal gluten, 
and exuded in strata from the parts of the body which enter 
last into the shell, namely, from the external swelling of the 
collar. There are, however, a certain number of species, 
which, even in our climates, do not produce this epiphragma. 
Probably, at this period, they bury themselves deeper in 
