which are retained in those forms in which the branchiæ are permanent (Perennibranchiata). In the tailed Amphibians (Urodela), and in the Frogs and Toads (Anoura) 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 (Caducibranchiata), and lowest in those in which the branchia are retained throughout life (Perennibranchiata).

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 branchiæ suffer a proportionate diminution in size. In those Amphibians in which branchiæ 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 branchiæ, 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. 257); whereas in the "Tail-less" forms, such as the Frogs (fig. 256), the tail is absorbed before maturity is attained. In a few cases, it seems questionable if the larvæ possess branchia, 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 metamor

phosis takes place, this is completed before the young animal begins to lead a free existence.

The endoskeleton of the Amphibia is generally well ossified,

B

A

Fig. 257.-Tailed Amphibians. A, Siren lacertina; B, Amphiuma, showing the four minute limbs; C, Menobranchus maculatus. (After Mivart.)

and the skull possesses two occipital condyles. The vertebræ are biconcave or amphicœlous (as in Fishes) in Proteus, Cæcil ians, and most of the extinct Labyrinthodonts. In the Salamanders and Surinam Toads the vertebræ are opisthocoelous, but most of the other Amphibians have procœlous vertebræ The length of the vertebral column is greatly reduced in the tail-less forms, and the number of vertebræ 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 Cæcilians, however, there are welldeveloped ribs, which are never supplemented in front by sternal ribs, though a cartilaginous or partially-ossified sternum may be present.

Limbs may be entirely wanting (as in the Cæcilians 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. 257)

The skin is mostly soft, moist, and richly provided with glands; the Cæcilians have mostly small rounded horny scales imbedded in the integument, and the extinct Laby

rinthodonts 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 larval 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, præmaxillæ, maxillæ, and mandible, and are generally disposed in the upper jaw in the form of two concentric semicircles. In the larvæ of the Frogs and Toads the front of the maxillæ 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.

CHAPTER LIX.

ORDERS OF AMPHIBIA.

THE Amphibia are usually divided by modern writers into four orders, the old order Lepidota, comprising the Lepidosiren, being now placed at the head of the Fishes, under the name of Dipnoi. Whilst there is a general agreement as to the number and characters of the Amphibian orders, the names employed to designate them are very various, and it really matters little which are adopted.

ORDER I. OPHIOMORPHA, Owen (= Gymnophiona, Huxley; Apoda, Peromela, Ophidobatrachia) :— Serpentiform or vermiform Amphibians, without limbs; anus terminal; skin mostly with horny scales imbedded in it. Eyes rudimentary or absent.

This is a small order, including only certain snake-like, vermiform animals (fig. 258) which are found in various tropical countries, burrowing in marshy ground, something like

gigantic earth-worms. They form the family Cæciliada (so called by Linnæus from their supposed blindness), and are

Fig. 258. Ophiomorpha. a Siphonops annulatus, one of the Cæcilians, much reduced: Head; c Mouth, showing the tongue, teeth, and internal openings of the nostrils; d Tail and cloacal aperture. (After Dumeril and Bibron.)

characterised by their snake-like form, and by having the anus placed almost at the extremity of the body. The body is cylindrical and worm-like, and is completely destitute of limbs. The skin is glandular, naked, and viscous, thrown into numerous folds, and containing numerous delicate, rounded, horny scales, which are dermal in their character, and are wanting in Siphonops annulatus. The mandibular rami are short, and are united in front by a symphysis. The teeth are long, sharp, and generally recurved; and a row of palatine teeth forms a concentric series with the maxillary teeth. The tongue is fleshy, fixed to the concavity of the lower jaw, and not protrusible (fig. 258). The ribs are numerous, but there is no sternum. The adult possesses lungs, one of which is smaller than the other, and the nose opens behind into the mouth. The eyes are rudimentary, nearly concealed beneath the skin, or altogether wanting.

The position of the Cæcilia was long doubtful; but their Amphibian character was ultimately proved by the discovery that whilst the adult breathes by lungs, the young possess internal branchiæ, communicating with the external world by a branchial aperture on each side of the neck. In addition to the presence of branchiæ in the larva, the Cæcilians are further connected with the Amphibia by their possession of a double