The class Amphibia comprises the Frogs and Toads, the Sala-mandroids, the Caeciliae, and the extinct Labyrinthodonts, and may be briefly defined as follows: - As is the case with the Fishes, the embryo is not furnished with an amnion, and the urinary bladder is the only representative of the allantois. As in Fishes, also, branchiae or filaments adapted for breathing air dissolved in water are always developed upon the visceral arches for a longer or shorter time. On the other hand, the Amphibians differ from the Fishes in the fact that true lungs are always present in the adult; the limbs are never converted into fins ; and when median fins are present, as is sometimes the case, these are never furnished with fin-rays. The limbs, when present, exhibit in their skeleton the same parts as do the limbs of the higher Vertebrates. The skull always articulates with the vertebral column by means of two occipital condyles. The heart consists of two auricles and a single ventricle. The nasal sacs communicate posteriorly with the pharynx ; and the rectum, ureters, and ducts of the reproductive organs open into a common chamber or "cloaca."
The great and distinguishing character of the Amphibia is the fact that they undergo a metamorphosis (rarely concealed or inconspicuous) after their exclusion from the egg. They commence life as water-breathing larvae, provided with gills or branchiae; but in their adult state they invariably possess lungs - the branchiae in the higher forms disappearing when the lungs are developed - but being in other cases permanently retained throughout life.
In the earliest embryonic condition the branchiae are external, placed on the side of the neck, and not situated in an internal chamber, as in Fishes. In some cases the external branchiae only are present, and they are, in any case, the gills which are retained in those forms in which the branchiae are permanent (Perennibranchiata). In the tailless Amphibians (Anoura), with hardly an exception, two sets of gills are developed - an external set, which is very soon lost, and an internal set, which is retained for a longer or shorter period. As maturity is approached, true lungs adapted for breathing air are developed. The development, however, of the lungs varies with the completeness with which aerial respiration has to be accomplished; being highest in those forms which lose their gills when grown up (Caducibranchiatd), and lowest in those in which the branchiae are retained throughout life (Perennibranchiata); while even in the highest forms of the class the structure of the lung is very simple.
Fig. 276. - Anoura. Hyla leucotaenia, one of the Tree-frogs (after Gunther).
In accordance with the change from an aquatic or branchial to a more or less completely aerial or pulmonary mode of respiration, considerable changes are effected in the course and distribution of the blood-vessels. In the larval condition, when the respiration is entirely effected by means of the gills, the circulation is carried on very much as it is in Fishes. The heart is composed of a single auricle and ventricle, and the blood is propelled through a bulbus arteriosus and branchial artery to the gills. The aerated blood is then collected in the branchial veins, and instead of being returned to the heart, is forthwith propelled to all parts of the body, the descending aorta being formed out of the branchial veins. At this stage, therefore, the heart is a branchial one, and the single contraction of the heart is sufficient to drive the blood through both the branchial and systemic circulations, just as we saw was permanently the case with all the Fishes except the Dipnoi. The pulmonary arteries are at first very small, and take their origin from the last pair of branchial arteries. When the lungs, however, are developed, and the respiration commences to be aerial, the pulmonary arteries increase proportionately in size, and more and more blood is gradually diverted from the gills and carried to the lungs, so that the branchiae suffer a proportionate diminution in size. In those Amphibians in which branchiae are permanently retained (Perennibranchiata), this state of affairs remains throughout life - that is to say, a portion of the venous blood is sent by the pulmonary artery to the lungs, and a portion goes to the gills. In those Amphibians, however, in which the adult breathes by lungs alone (Caducibranchiata), further changes ensue. In these the pulmonary arteries increase so much in size that they ultimately divert all the blood from the branchiae, and these organs, having fulfilled their temporary function, become atrophied and disappear. The vessels which return the aerated blood from the lungs (the pulmonary veins) increase in size proportionately with their increased work, and ultimately come to open into a second auricle formed at their point of union. The heart, therefore, of the Amphibia in their adult state consists of two auricles and a common ventricle. The right auricle receives the venous blood from the body, and the left receives the arterial blood from the lungs, and both empty their contents into the single ventricle. As in Reptiles, therefore, the ventricular cavity of the heart in adult Amphibians contains a mixed fluid, partly venous and partly arterial, and from this both the body and the lungs are supplied with blood.
The larval Amphibians are furnished with a more or less extensively developed caudal appendage or tail, which may or may not be retained throughout life. In the so-called "tailed" Amphibians, such as the Newts, the larval tail is permanently retained (fig. 277); whereas in the "Tail-less" forms, such as the Frogs (fig. 276), the tail is absorbed before maturity is attained. In a few cases, it seems questionable if the larvae possess branchiae, and there is no metamorphosis properly so called, since the young animal resembles the adult in all except size almost immediately after exclusion from the egg. In one of these cases (Hylodes) the larval tail appears to officiate as a breathing-organ, before emergence from the egg, but is absorbed within the first day after hatching. In other cases, again - e.g., in Pipa and Nototrema - though a metamorphosis takes place, this is completed before the young animal begins to lead a free existence.
The endoskeleton of the Amphibia is generally well ossified, and the skull possesses two occipital condyles. The vertebrae are biconcave or amphicoelous (as in Fishes) in Proteus, Caecil-ians, and most of the extinct Labyrinthodonts. In the Salamanders and Surinam Toads the vertebrae are opisthocoelous, but most of the other Amphibians have procoelous vertebrae. The length of the vertebral column is greatly reduced in the tail-less forms, and the number of vertebrae is correspondingly small. The sacrum is seldom composed of more than one vertebra, and there are often no separately-ossified ribs. In the Tailed Amphibians and the Caecilians, however, there are well-developed ribs, which are never supplemented in front by sternal ribs, though a cartilaginous or partially-ossified sternum may be present.
Fig. 277. - Tailed Amphibians. A, Siren lacertina; B, Amphiuma, showing the four minute limbs; C, Menobranchus maculatus. (After Mivart.)
Limbs may be entirely wanting (as in the Caecilians and some of the Labyrinthodonts); but all the other members of the class possess both pairs of limbs, with the exception of the genus Siren, in which the pelvic limbs are wanting (fig. 277).
The skin is mostly soft, moist, and richly provided with glands; the Caecilians have mostly small rounded horny scales imbedded in the integument; and the extinct Labyrinthodonts possessed a ventral armour of bony scales. Integumentary ossifications are also developed in some other cases (e.g., Ceratophrys).
As regards the digestive system of the Amphibia there is little to say, except that the rectum opens, as it does in Reptiles, into a common chamber or "cloaca," into which are also discharged the secretions of the kidneys and generative organs. A liver, gall-bladder, spleen, and pancreas are always present. Singular pulsating cavities, belonging to the lymphatic system, and known as "lymph-hearts," are also present in the higher Amphibians. The alimentary canal is much longer in the lar-val Amphibians than in the adult. A tongue may or may not be present, but there are no salivary glands. Teeth are. usually developed in the vomer, praemaxillae, maxillae, and mandible, and are generally disposed in the upper jaw in the form of two concentric semicircles. In the larvae of the Frogs and Toads the front of the maxillae and mandible are encased in a horny beak.
A urinary bladder is present, opening into the cloaca, and there are well-developed kidneys. The ducts of the reproductive organs communicate with the urinary ducts. The ova are usually impregnated outside the body; but internal impregnation occurs in some of the Urodela.