Page:The New International Encyclopædia 1st ed. v. 01.djvu/563

AMPHIARAUS. nices. As he fled from the victorious Thebans, Zeus caused the earth to open and en^ilf Am- phiaraiis with his horses and chariot. He was a descendant of the se£r Melaniinis, and son of Oleics and Hj-permnestra. According to hiter writers, he took part in the Calydonian hunt and the Argonautic expedition.

AMPHIB'IA (Gl<. ift0i. amphi, on both sides -j- /^lof, hius, life). A class of vertebrates intermediate between fishes and reptiles. It was made by Linnaeus to comprise rci)tiles, amphibia, and cartilaginous fishes, but has been restricted, until now it is equivalent to Batrachia, and in- cludes frogs, toads, newts, salamanders, the snaUc-like Gymno])hiona and gigantic e.xtinct forms, the Stegocephali. As adults, many, but not all of them, are able, either by the possession of lungs or by means of skin respiration, to come from water to land; hence their name. Distinctive C'h.racteristic.s. All the free- developing amphibia possess gills in the larval stage that, in some forms, persist throughout life. The skin is soft and glandular, and serves in part or wholly to aerate the blood. The outer layers of the skin become cornificd and are periodically shed, and in a few cases there is a bony dermal skeleton. The paired fins of fishes are replaced by pentadactyl legs. In some forms, as in sirens, one pair of limbs may be wanting, and in such forms as G3'mnophiuna both pairs may be lacking. Tlie mouth is terminal and the teeth are firmly ankylosed to the supporting bones. The tongue, when present, is bifid, and is so fixed at the front of the moutli that the free end turns backward. The mouth and pharynx are ciliated and into them open the internal nares. The alimentary tract is nearly straight in the elongated forms, or it may be much convoluted, as in the case of vegetable feeding tadpoles. There is a two-lobcd liver and a pancreas. The lungs are thin-walled sacks "that may have internal folds, but some salamanders are lungless. The heart is usually composed of two distinct auricles, one ventricle, and a conus arteriosus. The red blood-corpuscles are oval, nucleated, and large in comparison with those of warm-blooded vertebrates.

BuEEmNG Hauits. The eggs may be fertilized internally or externally, just as they are being de]iositcd in the water. In most cases they are left to chance, but in some species are carried in strands, or otherwise cared for by the male or female. A few have brood-pouches, and one toad rears its young in pits in the skin of the back. A few forms bring forth their young in an active condition. The gill-bearing or larval stage (a.xolotl) of Amblystoma tigrinum is capable of breeding, and under certain conditions may un- dergo its metamorphoses. (See AxoLOTL.) The eggs are pigmented and usually undergo total and unequal segmentation. They possess a large amount of yolk, so much in a few eases, siich as pipa, that the embryo lies coiled over the egg as though it were a fish. The blastula and gastrula stages are present, but are modified in form and manner of development by the pres- ence of the yolk; the medullary groove develops by a pair of upfoldings along "the middle of the back, and by fusion of head and body, the tail becomes marked ofT; on the neck are two or three pairs of external gills. At about this time the tadpoles hatch, and begin to swim about or adhere to weeds by means of the sucker on the ventral surface of the head. At first the tadpole has no mouth, but soon one develops, the external gills dwindle and are replaced by the internal, yhich are covered by a fold of .skin. The liind limbs are the first to appear externallj-, lungs develop, and the larva can breathe both on land and in water. The gills of the Anura continue to dwindle and likewise the tail is gradually and completely absorbed. Tadpoles as well as some adult amphibia have the power of reproducing lost parts. See Toad.

The early stages of amphibians are not al- ways passed in water. Some of the European salamanders are viviparous, the j"oung beincf born ajl developed, but still requiring water. The young of the viviparous Ca>ciliidir, however, take to a terrestrial life as soon as they are born. So, too, certain frogs (e.g. Kana opi'sthodon, of the Solomon Islands) hatch from eggs laid out of the water as perfect, air-breathing frogs. In many species, as in the persistent gilled Crodela, the adult lives chiefly in the water; in other cases, as in the other Urodela, the CseciliidiE, and the Anura, the adult lives on land. Habits. The adult Amphibia feed on worais, slugs, and insects. Hence they are all useful to agriculture. Kone has a poisonous bite, but all trust largely for safety to acrid or poisonous secretions from the skin-glands. The tadpoles subsist almost entirely on water vegetation, such as algae. In cold or dry seasons Amphibia pre- serve themselves by burrowing down into mud and earth, and there fall into a lethargic sleep. Jlost Amphibia keep near water, and their young develop in it. A few forms that live in moun- tains, in trees, or on dry, porous volcanic islands, bring forth their young well enough developed to breathe air, thus approaching a reptilian con- dition of development. Many, but not all, am- phibians are nocturnal, being most active in their search for food or mates in the early morning or evening hours.

Geographical Di.stribution. The amphibia thrive best in warm and moist countries. A few live in the temperate zone, some frogs penetrat- ing far north, but not so far as the polar regions. The order of relative abundance of amphibia in the difl'erent countries is as follows: Tropical America, India, Afi'ica, Australia, North Amer- ica, Europe. Many families and genera have a very limited range, since, although fresh water is a necessity to them, the sea is a complete barrier to their spread. Salamanders are con- fined mainly to Europe and North America, and only toads and frogs are of world-w-ide distri- bution.

Classification. There are four orders of Amphibia: the Urodela (q.v.), possessing a tail throughout life (newts and salamanders) ; the Anura (q.v,), without tail in the adult stage (frogs and toads); the Gymnophiona (q.v.), snake-like, without limbs, and blind, and the Stegocephalia (q.v.), and other extinct often gigantic tailed forms fossil in the Carboniferous, Permian and Trias rocks. The existing species number about 1000.

Ancestry. The Amphibia have doubtless sprung from fish-like ancestors, and the link with that ancestry is found in the fossil group of Stegocephalia, 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 Crossopterygii. Gadow, in 1901, gave the following features of Amphibia as those that proclaim their piscine descent: (1) The possession by