ANCESTRY. The Amphibia have doubtless sprung from fish-like ancestors, and the link with that ancestry is found in the fossil group of Stegoeephalia, whose head carries great plates. The piscine group from which the Amphibia arose must have been either the Dipnoi, which are to-day largely air breathers, or the Cros sopterygii. Gadow, in 1901, gave the following features of Amphibia as those that proclaim their piscine descent: (1) The possession by the heart of a long eonus arteriosus (anterior to the ventricle) provided with, in many cases, numerous valves, on at least (in Anura) one series at the base, another at the beginning of the truncus where the arches branch off; (2) the strictly symmetrical arrangement of these arches; (3) the three-chambered heart is still like that of Dipnoi; (4) the occurrence of as many as four or even five branchial skeletal arches in the larval stage; (5) the glottis (or entrance to windpipe) is supported by carti lages which themselves are derivatives of pos terior visceral arches; (6) the development (in Urodela as in Stegocephalia) of the verfebrce from four pairs of elements called arcvalia, and the formation of the intervertebral joints by a split across the intervertebral ring of cartilage; (7) the hypoglossal nerve still lies outside and be hind the skull as a cranial nerve; (8) the pres ence of lateral sense organs; (9) the possession of external gills as in Dipnoi and CrossoptAygii. It is frequently assumed that the first Urodela were aquatic creatures, provided with a finned tail and small lungs. Gadow believes these to be larval -acquisitions. not ancestral reminis cences. The fact that the ancestors of Amphibia evolved the pentadaetyl condition proves that they were land animals. The evolutionary change through which the early Amphibia passes are thus enumerated by Gadow.
(1 ) "Terrestrial, with two pairs of pentadac tyloid limbs; breathing by lungs only; with a fully developed apparatus of five pairs of gill arches, which during the embryonic life perhaps still carried internal gills, with or without sev eral pairs of gill clefts. Reduction of the dermal armor and of the cutaneous scutes had taken place.
(2) "Additional respiratory organs were de veloped by the embryo, in the shape of external gills; these were at first restricted to embryonic life (as in the existing Apoda), but were grad ually used also during the aquatic life of the larva. These external gills, together with the lungs, have superseded the internal gills, of which there are now no traces either in Urodela or in Anima.
(3) "Some Urodela, retaking to aquatic life, retained and further enlarged the external gills into more or less permanent organs. The majority of Urodela hurried through the larval, aquatic stage, and some—e.g., Salamandra atra —became absolutely terrestrial. The possession of unusually long external gills by this species and by the Apoda indicates that these organs are essentially embryonic, not larval, features." BIBLIOGRAPHY: The foremost systematic Bibliography: The foremost systematic writers upon this group are G. A. Boulower, of the British Museum, and E. D. Cope. The latter has completely monographed North American forms in "Batraehia of North America." Bulletin d.fi, United States National Museum (Washing ton, 1889). This discusses the larger relations of the group, and gives an extensive bibliography. For a still more recent general treatise, consult H. Gadow, "Amphibia." Cambridge Natural His tory, Volume VIII. (Cambridge, 1901). See ALIMENTARY SYSTEM (Evolution of) and simi lar articles relating to comparative anatomy.