15. Development takes place without amnion and allantois.
None of these characters is absolutely diagnostic, except 1 (c), and this applies only to the Anura and most of the Stegocephali.
Numbers 1 (b), 1 (c), 2, 3, 4 and 12 separate the Amphibia from the Fishes.
Numbers 1, 6 (b), 7, 8, 9, 11, 13, 15 separate them from the Reptiles, Birds, and Mammals.
Number 2 separates them from the Fishes, Reptiles, and Birds.
Number 5 separates them from the Mammals.
Number 6 (a) separates them from the Fishes (excl. Dipnoi), Birds and Mammals.
We can, therefore, very easily define all the Amphibia, both recent and extinct, by a combination of the characters enumerated above. For instance, by the combination of numbers 2, 3 or 4 with either 7, 8, 9, 11, 13 or 15.
Amphicondylous Anamnia would be an absolutely correct and all-sufficient diagnosis, but it would be of little use in the determination of adult specimens; and the tetrapodous character is of no avail for Apoda. Amphicondylous animals without an intracranial hypoglossal nerve is a more practical diagnosis.
In the case of living Urodela and Anura the absence of any scales in the skin affords a more popular character; it is unfortunately not applicable to the Apoda, many of which possess dermal scales, although these are hidden in the imbricating transverse rings of the epidermis; and the frequent occurrence of typical scales of both ecto- and meso-dermal composition in many of the Stegocephali forces us to discard the scales, or rather their absence, as a diagnostic character of the class Amphibia. The same applies to the mostly soft, moist, or clammy, and very glandular nature of the skin.