As for the properties of the internal organs, these we must first
discuss in the case of the animals that are supplied with blood. For
the principal genera differ from the rest of animals, in that the
former are supplied with blood and the latter are not; and the
former include man, viviparous and oviparous quadrupeds, birds,
fishes, cetaceans, and all the others that come under no general
designation by reason of their not forming genera, but groups of which
simply the specific name is predicable, as when we say 'the
serpent,' the 'crocodile'.