XVl INTRODUCTION. 
separate. Some are viviparous ; some oviparous, and their 
eges are enveloped in a shell composed of a simple visco- 
sity, which is more or less hard. 
All the varieties relative to digestion or generation are 
generally all found in the same orders, and sometimes only 
in the same family. 
And lastly, the mollusca in general are animals but little 
developed, little susceptible of mdustry: all of them are 
remarkable for their fecundity and tenacity of hfe; and 
their symmetry is less complete than that of the vertebrose 
animals. 
