But among

insects and fishes, some cases are found wholly devoid
of this duality of sex. For instance, the eel is neither male nor
female, and can engender nothing. In fact, those who assert that
eels are at times found with hair-like or worm-like progeny
attached, make only random assertions from not having carefully
noticed the locality of such attachments. For no eel nor animal of
this kind is ever viviparous unless previously oviparous; and no eel
was ever yet seen with an egg. And animals that are viviparous have
their young in the womb and closely attached, and not in the belly;
for, if the embryo were kept in the belly, it would be subjected to
the process of digestion like ordinary food. When people rest
duality of sex in the eel on the assertion that the head of the male
is bigger and longer, and the head of the female smaller and more
snubbed, they are taking diversity of species for diversity of sex.