MOLLUSCA. 449 



iThe former are in tlieir 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 Eveeard Home, 

 when treatinjr of the disting-uished 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 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."" 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 5di August 1773, Avere hatched on the 20th of that 

 month, and their condition at this time distinctly stated, 



VOL. II. F f 



