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.
Text Book of Biology, Part 1: Vertebrata · The Wunder Library — complete classics, free to read, with narration.