Not only have the invertebrata, as shown by geological data, altered at a less rapid rate than the vertebrata, but if we take one of the classes of the former, as for example the mollusca, we find those of more simple structure to have varied at a slower rate than those of a higher and more complex organisation; the Brachiopoda, for example, more slowly than the lamellibranchiate bivalves, while the latter have been more persistent than the univalves, whether gasteropoda or cephalopoda.
"The Student's Elements of Geology"
Sir Charles Lyell
In the next higher class of the gasteropoda, or marine univalve shells, the sexes are either united or separate.
"The Descent of Man and Selection in Relation to Sex, Vol. I (1st edition)"
Charles Darwin