With regard

to animals in general, some parts or organs are
common to all, as has been said, and some are common only to
particular genera; the parts, moreover, are identical with or
different from one another on the lines already repeatedly laid
down. For as a general rule all animals that are generically
distinct have the majority of their parts or organs different in
form or species; and some of them they have only analogically
similar and diverse in kind or genus, while they have others that
are alike in kind but specifically diverse; and many parts or organs
exist in some animals, but not in others.