With regard to sex,

some animals are divided into male and female,
but others are not so divided but can only be said in a comparative
way to bring forth young and to be pregnant. In animals that live
confined to one spot there is no duality of sex; nor is there such, in
fact, in any testaceans. In molluscs and in crustaceans we find male
and female: and, indeed, in all animals furnished with feet, biped
or quadruped; in short, in all such as by copulation engender either
live young or egg or grub. In the several genera, with however certain
exceptions, there either absolutely is or absolutely is not a
duality of sex. Thus, in quadrupeds the duality is universal, while
the absence of such duality is universal in testaceans, and of these
creatures, as with plants, some individuals are fruitful and some
are not their lying still