CHAPTER XXXI.

ΑΜΡΗΙΒΙΑ.

THE class Amphibia comprises the Frogs and Toads, the Salamandroids, the Cæcilia, and the extinct Labyrinthodonts, and may be briefly defined as follows:-As is the case with the Fishes, branchia, 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 after their exclusion from the egg. They commence life as water-breathing larvæ, provided with gills or branchiæ; but in their adult state they invariably possess lungs; the branchiæ 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 branchiæ are external, placed on the side of the neck, and not situated in an internal chamber as in Fishes. In some cases the external branchiæ only are present, and they are, in any case, the gills which are retained in those forms in which the branchia 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 branchiæ are retained throughout life (Perennibranchiata).

The class Amphibia is divided into the four orders of the

Ophiomorpha, Urodela, Anoura, and Labyrinthodontia. The first of these includes only the serpentiform animals known as Cæcilia, and not having any certain fossil representatives, may be altogether passed over here. The order Urodela comprises the so-called "tailed" Amphibians of the present day, such as the Newts and Salamanders. The earliest traces of this order in past time occur in the Tertiary deposits. The order Anoura includes the so-called " tail-less" Amphibians, such as the Frogs and Toads, and is likewise not known to have existed in periods anterior to the Tertiary. Lastly, the order Labyrinthodontia is entirely extinct, and is known to have existed only during the Carboniferous, Permian, and Triassic periods.

ORDER I. URODELA (= Ichthyomorpha, Owen; Saurobatrachia). This order is commonly spoken of collectively as that of the "Tailed" Amphibians, from the fact that the larval tail is always retained in the adult. The Urodela are characterised by having the skin naked, and almost invariably destitute of any exoskeleton. The body is elongated posteriorly to form a compressed or cylindrical tail, which is permanently retained throughout life. The dorsal vertebræ are biconcave (amphicalous), or concave behind and convex in front (opisthocælous), and they have short ribs attached to the transverse processes. The bones of the fore-arm (radius and ulna) on the one hand, and those of the shank (tibia and fibula) on the other, are not anchylosed to form single bones.

The best known of the existing Urodela are the Newts (Triton), the Salamanders (Salamandra), the Mud-eels (Siren), the Axolotl (Siredon), and the Giant-Salamanders (Menopoma). Some of these are "perennibranchiate," retaining the branchia throughout life; others lose the branchiæ, becoming thus "caducibranchiate," but retain the branchial apertures behind the head; others, lastly, lose both the branchia and the bran chial apertures. Most of the Urodela have the four limbs well developed, but some possess only the anterior limbs. The geological history of the Urodela is short and of little. importance. No trace of the order has hitherto been discovered

in

any deposits older than the Tertiary. The only exception to this statement is constituted by the fossil described from the Lower Permian Rocks by Geinitz under the name of Palæosiren, and regarded by him as being most nearly allied to the Siren lacertina. It is probable, however, that Palæosiren is really referable to the Labyrinthodontia. In strata of Tertiary age have been discovered the remains of Newts and Salamanders. The most remarkable fossil referable to this order is the Andrias Scheuchzeri (fig. 307) of the Miocene beds of

Oeningen. This singular fossil was described by its original discoverer as human, under the name of Homo diluvii testis; but it is really the skeleton of a Salamandroid of large size. It is very closely allied to the Giant Salamander (Menopoma, or Sieboldia, maxima) of Java.

ORDER II. ANOURA (= Batrachia, Huxley; Theriomorpha, Owen; Chelonobatrachia, &c.)-This order includes the Frogs and Toads, and is perhaps best designated by the name of Anoura, or "Tail-less" Amphibians. The name Batrachia, employed by Huxley, is inexpedient, partly because it is used by Owen to designate the entire class Amphibia, and partly because, in common language, it is usual to understand by a "Batrachian" any of the higher Amphibians; such, for instance, as a Labyrinthodont.

The Anoura, or Tail-less Amphibians, are characterised by the following points :-The adult is destitute of both gills and tail, both of which structures exist in the larva, whilst the two pairs of limbs are always present. The skin is soft, and there are rarely any traces of an exoskeleton. The dorsal vertebræ are "procœlous" or concave in front, and are furnished with

d

M

Fig. 308.-Skeleton of the common Frog (Rana temporaria). d Dorsal vertebræ, with long transverse processes.

long transverse processes, which take the place of ribs, which are only present in a rudimentary form. The radius and ulna in the fore-limb, and the tibia and fibula in the hind-limb, are

anchylosed to form single bones (fig. 308). The mouth is sometimes edentulous, but the upper jaw has usually small teeth, and the lower jaw sometimes. The hind-limbs usually have the digits webbed for swimming, and are generally much larger and longer than the fore-limbs.

The geological history of the Anoura, as in the case of the Urodela, is of small importance. The two chief groups of the living Anoura-namely, the Frogs and the Toads-are both represented in past time; but they do not appear to have come into existence till after the commencement of the Tertiary period. Most of the fossil forms have been detected in deposits of Miocene age.

ORDER IV. LABYRINTHODONTIA. -The members of this, the last order of the Amphibia, are entirely extinct. They were Batrachians, probably most nearly allied to the Urodela, but all of large size, and some of gigantic dimensions, the skull of one species (Labyrinthodon Jægeri) being upwards of three feet in length and two feet in breadth. The Labyrinthodonts were first known to science simply by their footprints, which were found in certain sandstones of the age of the Trias. These footprints consisted of a series of alternate pairs of hand-shaped impressions, the hinder print of each pair being much larger than the one in front (fig. 309). So like were these impressions to the shape of the human hand that the unknown animal which produced them was at once christened Cheirotherium, or "Hand-beast." Further discoveries, however, soon showed that the footprints of Cheirotherium had been produced by different species of Batrachians, to which the name of Labyrinthodonts was applied in consequence of the complex microscopic structure of the teeth.

The order Labyrinthodontia is thus defined by Professor Huxley :-"The body is salamandriform, with relatively weak limbs and a long tail. The dorsal vertebræ, when completely ossified, are biconcave, with double transverse processes. The ribs have distinct capitula and tubercula.

"In the thoracic region, three superficially sculptured exoskeletal plates, one median and two lateral, occupy the place of the interclavicle and clavicles. Between these and the pelvis is a peculiar armour, formed of rows of oval dermal plates, which lie on each side of the middle line of the abdomen, and are directed obliquely forwards and inwards, to meet in that line.

"The skull has distinctly ossified epiotic bones, in the same position and of the same form as those of fishes. The cranial bones are sculptured, and many exhibit peculiar smooth symmetrical grooves-the so-called 'mucous canals.'