MOLLUSCA. 449 



The former are in their natural state, the latter ought to 

 be regarded as monsters. Where the character is perma- 

 nent, 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 pro- 

 portional size previous to hatching. Sir EVERARD HOME, 

 when treating of the distinguished characters between the 

 ova of the sepia, and those of the vermes testacea that live 

 in water (Phil Trans. 1817 3 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 co- 

 vering, 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 testacea that live in water, the young requires some de- 

 fence in the period between the egg being hatched and the 

 young acquiring its shell, which is not necessary in those 

 that live on land ; for this purpose, the ova are enclosed 

 in chambers of a particular kind. 11 The assertion here 

 made, and founded on a priori considerations, that the 

 shell is not formed until after the egg is hatched, is oppos- 

 ed by every observation which we have been able to make 

 on the subject ; and what is more surprising, it is at vari- 

 ance with his own observations on the garden snail, the very 

 example produced in its confirmation. The eggs of a snail, 

 laid on 5th August 1773, were hatched on the 20th of that 

 month, and their condition at this time distinctly stated*' 



VOL. n. F f 



