148 MOLLUSCA- 
ever, all the organs preserve the same relation to the back. 
and belly, the head and tail. It is impossible, therefore, to 
conceive a dextral animal changed into a sinistral, by any 
circumstance which could take place at the period of hatch- 
ing, as M. Bosc was inclined to believe. This arrangement 
of the organs must have been not merely congenital, but 
coeval with the formation of the embryo. In some species 
all the individuals are sinistral, while in others the occur- 
rence is rarely met with in a solitary example. The former 
are in their natural state, the latter ought to be regarded as 
monsters. Where the character is permanent, it should 
constitute a generical difference. 
The reproductive system of the animals of this class ex- 
hibits the sexual organs, in general, united in the same in- 
dividual. Mutual impregnation, however, is necessary. All 
the species are oviparous. The eggs are either naked, as 
in the terrestrial genera, or enveloped in a gelatinous mass, 
like the aquatic kinds. The embryo acquires nearly all its 
members while in the egg, and the shell is of a proportional 
size previous to hatching. Sir Everard Home, when treat- 
ing of the distinguished characters between the ova of the 
sepia, and those of the vermes testacea that live in water, 
(Phil. Trans., 1817, p. 297), and when referring to the ova 
of the vermes testacea, says, “If the shell were formed in 
the ovum, the process of aérating the blood must be very 
materially interfered with, for this reason, the covering or 
shell of the egg, first drops off; and the young is hatched 
before the shell of the animal is formed; this I have seen 
take place in the eggs of the garden snail, but in the tes- 
tacea that live in water, the young requires some defence in 
the period between the egg being hatched and the young 
acquiring its shell, which is not necessary in those that live 
