vertebrata, except the true fishes, which do not possess any of them--
(a) Lungs (but many fish have a swimming bladder which answers
to the lungs in its anatomical relations.)
(b) Limbs which consist of a proximal joint of one bone an
intermediate part of two, and a distal portion which has five
digits, or is evidently a reduced form of the five-digit limb.*
(c) The absence of a median fin supported by fin rays.**
* The frog shows indications of a sixth digit.
** The frog's tadpole has a median fin, but no fin rays.