| 1 | a. Fins present b. Fins absent |
2 3 |
| 2 | a. Jawless; several gill openings behind head b. Bony jaws present; usually a single gill opening behind head covered by an operculum |
Agnatha Osteichthyes |
| 3 | a. Skin naked, with no protective coverings b. Skin with scales or other protective coverings |
Amphibia 4 |
| 4 | a. Skin with scales; feathers or hair not present b. Skin without scales; either feathers or hair present |
Reptilia 5 |
| 5 | a. At least some part of body with feathers b. At least some part of body with hair |
Aves Mammalia |