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 aerating 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 



