Amphibia

Encyclopaedia Britannica, 9th ed. (1878)

[750] Linnæus originally employed this term to denote a class of the Animal Kingdom comprising crocodiles, lizards, and salamanders snakes and Cæciliæ, tortoises and turtles, and frogs; to which, in the later editions of the Systema Naturæ, he added some groups of fishes. In the Tableau Elémentaire, published in 1795, Cuvier adopts Linnæus's term in its earlier sense, but uses the French word "Reptiles," already brought into use by Brisson, as the equivalent of Amphibia. In addition, Cuvier accepts the Linnæan subdivisions of Amphibia-Reptilia for the tortoises, lizards (including crocodiles), salamanders, and frogs; and Amphibia-Serpentes for the snakes, apodal lizards, and Cæciliæ.

In 1799¹ Brongniart pointed out the wide differences which separate the frogs and salamanders (which he terms Batrachia) from the other reptiles; and in 1804, Latreille,² rightly estimating the value of these differences, though he was not an original worker in the field of vertebrate zoology, proposed to separate Brongniart's Batrachia from the class of Reptilia proper, as group of equal value, for which he retained the Linnæan name of Amphibia.

Cuvier went no further than Brongniart, and, in the Règne Animal, he dropped the term Amphibia, and substituted Reptilia for it. Meckel,³ on the other hand, while equally accepting Brongniart's classification, retained the term Amphibia in its earlier Linnæan sense; and his example has been generally followed by German writers; as, for instance, by Stannius, in that remarkable monument of accurate and extensive research, the Handbuch der Zootomie (Zweite Auflage, 1856).

In 1816 De Blainville,⁴ adopting Latreille's view, divided the Linnæan Amphibia into Squamiféres and Nudipelliféres, or Amphibiens; though he offered an alternative arrangement, in which the class Reptilia is preserved and divided into two sub-classes, the Ornithoides and the Ichthyoides. The latter are Brongniart's Batrachia, plus the Cæciliæ, whose true affinities had, in the meanwhile, been shown by Dumeril; and, in this arrangement, the name Amphibiens is restricted to Proteus and Siren.

[...] Professor Huxley has adopted Latreille's view of the distinctness of the Amphibia, as a class of the Vertebrata, co-ordinate with the Mammalia, Aves, Reptilia, and Pisces; and the same arrangement is accepted by Gegenbaur and Haeckel. In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Professor Huxley divided the Vertebrata into Mammals, Sauroids, and Ichthyoids, the latter division containing the Amphibia and Pisces. Subsequently he proposed the names of Sauropsida and Icthyopsida for the Sauroids and Ichthyoids respectively. It is proper to mention, finally, that Professor Owen, in his work on The Anatomy of Vertebrates, follows Latreille in dividing the Vertebrata into Hæmatotherma and Hæmatocrya, and adopts Leuckart's term of Dipnoa for the Amphibia.

The Amphibia are distinguished from the Sauropsida and Mammalia by very important and sharply-defined characters. The visceral arches of the embryo develop gills, which temporarily, or permanently, perform the respiratory function. There is no trace of an amnion, and it is still a question whether the urinary bladder, which all Amphibia possess, answers to the allantois of the higher Vertebrata or not. At any rate, it plays no part in the respiration of the embryo, nor is it an organ by which nutriment is obtained from the parent. There are two occipital condyles, and the basi-occipital region of the skull is either very incompletely, or not at all, ossified. There is no basi-sphenoidal ossification. When young, the Amphibia are provided with, at fewest, three, and usually four, cartilaginous, or more or less ossified, branchial arches. From Pisces, on the other hand, they are distinguishable only by the characters of their locomotive apparatus. When they possess median fins and limbs, these never present fin-rays; and the limbs exhibit, in full development, the type of structure which obtains among the Sauropsida and Mammalia, and differ very widely from the fins of any fish at present known. This difference obtains even among [751] the long extinct Amphibia of the Carboniferous epoch. In other respects, the lower Amphibia approach the Chimæræ, the Ganoidei, and the Dipnoi very closely; while, in their development, they present curious approximations to the Marsipobranchii.

With respect to the primary subdivisions, or orders, of the class Amphibia, no one can doubt the propriety of the separation of the recent forms into what may be broadly termed Newts (Urodela); Frogs and Toads (Anura); and Cæciliæ (Peromela) effected by Dumeril; while all that is known of the organisation of the extinct Amphibia of the newer Palæozoic, and older Mesozoic, formations tends to show that they form a fourth natural assemblage of equal value to each of the others.

The names of Urodela and Anura, given to the first two of these divisions, are undoubtedly open to criticism; but if well-understood terms which have acquired a definite scientific connotation, are to be changed, whenever advancing knowledge renders them etymologically inappropriate, the nomenclature of taxonomy will before long become hopelessly burdened; and, to set a good example, the names of Urodela, Anura, Peromela, and Labyrinthodonta are adopted here for the four orders of the Amphibia, even although it be true that the Labyrinthodonta do not all possess the dental structure on which the name was founded; though there is reason to believe that some Labyrinthodonts were devoid of limbs, or peromelous; that the Anura are not more tail-less than are the Peromela; and that the tails of Urodela are not more conspicuous that were those of the Labyrinthodonts.

The Urodela are Amphibia with elongated bodies and relatively short limbs, devoid of scales or pectoral plates, with numerous præ-caudal vertebræ, and with amphicœlous, or opisthocœlous, vertebral centra. The hyoidean arch remains connected with the suspensorium throughout life, and its cornua are large in proportion to its body. The mandible is dentigerous. There are one or two pairs of limbs, the pectoral arch and limbs being always present. The manus never possesses more than four digits. The bones of the antebrachium and of the crus remain distinct, and the tarsus is not elongated. So far as the spermatozoa are known, they are elongated filaments with a vibratile fringe. The larva develop external gills only; and, except, none are known to possess, at any time, a horny masticatory apparatus.⁸

The Anura have relatively short and broad bodies, and both pairs of limbs are constantly present, the hinder being the longer and stronger. There are no scales, nor pectoral plates, but ossification sometimes occurs in the dorsal integument. The præ-sacral vertebræ never exceed nine in number, and the caudal portion of the vertebral column is represented by a peculiar styliform coccyx. [...}

The Peromela have snake-like bodies, totally devoid of limbs and limb arches. In most, the integument is provided with transverse rows of imbedded cycloid scales, but there are no pectoral plates The vertebræ of the trunk are very numerous, and are amphicœlous; those of the caudal region are very few, and are free. [...]

The Labyrinthodonta for the most part resemble the Urodela in the proportions of the tail and limbs to the body, but some (as Ophiderpeton) were serpentiform, and apparently apodal; no raniform Labyrinthodonts have yet been discovered. The vertebræ are amphicœlous. The mandible is dentigerous. The bones of the antebrachium and crus remain distinct, and the tarsus is not elongated. The manus and pes appear to have pentadactyle. Three sculptured pectoral plates and a peculiar dermal armour of small scales, confined to the ventral face of the body, are present in many genera. Nothing is known of the early stages of development, but the young Archegosauria appear to have possessed ossified branchial arches.

In giving a sketch of the organization of the Amphibia, it will be necessary to enter much more fully into the characters of the skeleton than into those of the other systems of organs.

The Vertebral Column, Ribs, and Sternum.–[...] Ribs are present in a few Anura, in all Urodela, Peromela, and Labyrinthodonta; and, in the last-named and some Urodela, they attain as great relative dimensions as in other Vertebrata. But they are always vertebral ribs, no Amphibian being known to possess more than rudiments of sternal ribs. The atlas is never provided with ribs.

[...]

[753] The Cranium.–The skull is always depressed, and is usually broad in proportion to its length, though, in this respect, there is considerable variation, the skulls of Proteus, Menobranchus, and Amphiuma being narrow, when compared with those of Siredon, Menopoma, and the Anura. The occipital foramen is situated in the middle of the posterior face of the cranium, and there are always two occipital condyles. [...]

[755] The hyoidean apparatus of the adult frog (Fig. 8) presents a body and two slender cornua. The body consists of a broad and thin squarish plate of cartilage, produced on each side into three processes, which may be called anterior, lateral, and posterior. The anterior process (a) is slender, curves outwards, and very soon divides into two processes, one short, anterior, forming a loop by its ligamentary connection with the second, or posterior, branch, which passes into the long and slender cornu of the hyoid. The lateral process (b) passes outwards and slightly dorsal–expanding into a broad, hatchet-shaped free extremity. The posterior process (c) is a mere prolongation of the postero-lateral angles of the body of the hyoid. Finally, from the middle of the posterior margin of the body of the hyoid there project two strong bony rods, wider at the ends than in the middle, which embrace the larynx, and have been termed the thryo-hyals (d). [...]

Fig. 8.–Ventral view of the hyoid of Rana esculanta.
a, anterior, b lateral, c, posterior processes; d, thryro-hyals.

[761] The Limbs.–The pectoral arch in the Amphibia is distinguishable into a scapular, a coracoidal, and a præcoidal region, although the extent to which these parts of the primitive cartilaginous arch become separately ossified varies very much in the different members of the group. [...]

[762] The Integumentary Organs.–In all recent Amphibia, the integument is remarkable for the great abundance of simple follicular glands which are distributed through it, and are sometimes all of one kind (e.g., Proteus), though in other cases two sorts of such glands can be distinguished (Rana). In many Anura and Urodela, these glandular structures attain a greater complication of structure, especially near the angle of the jaw, and constitute what are termed the "parotoid" glands. In some cases, the secretion of these glands is extremely acrid and irritating. In some Urodela (Proteus and Sirendon), and in the tadpole the epidermis becomes modified in relation with the termination of sensory nerves, in the head and along the body, in the region of the nerve of the lateral line, and gives rise to sensory organs of the same nature as those which are found in the lateral line and the so-called mucous sacs and canals of fishes. [...]

The Alimentary Organs.–The teeth of the recent Amphibia vary a good deal in form. In the Urodela, they are usually conical and pointed; frequently more or less curved; sometimes, as in Anaides, lancet-shaped. Siren has the surfaces of the vomera and palatines covered with parallel series of small dents en brosse. In Ceratophrys, the bases of the teeth are slightly grooved longitudinally. In Archegosaurus, similar grooves are more marked, and give rise to folds of the wall of the tooth. These, extending inwards and ramifying, give rise to the complicated or "labyrinthic" structure exhibited by transverse sections of the teeth of the typical Labyrinthodonts. Very generally, the teeth become ankylosed with the subjacent bones, and are replaced by others developed at their bases. In the Labyrinthodonts, some of the anterior teeth frequently become much larger than the rest. The Anura are remarkable for the total absence of teeth in the mandibles in all but one or two genera, while many have no premaxillary or maxillary teeth. The Toads have no teeth in the upper jaw. Pipa is altogether edentulous. Siren alone presents plates of horn upon the gingivel surfaces of the premaxillæ and of the dentary elements of the mandible. [...]

[763] The Organs of Circulation.–The heart is contained within a pericardium, the walls of which generally exhibit numerous scattered pigment cells, and though delicate in the fresh state, are apt to become tough and almost pergamentaceous in spirit specimens. The heart (if we may apply that name to the whole apparatus enclosed with the pericardium, except the venæ cavæ) presents a series of five segments [...]

Fig. 15.–The heart of Siredon mexicanus.

[766] The Respiratory Organs.–The glottis in the Amphibia is situated in the middle line of the floor of the pharynx. In the perennibranchiate Urodela, it is a very small longitudinal slit leading into a narrow passage, which widens into a chamber into which the elongated pulmonary sacs open. The Urodela and the Peromela present mere cartilaginous rudiments of a larynx; but, in the Anura, this structure attains a great development, and becomes the instrument of the powerful voice with which many of these animals are provided. The larynx is lodged in the angle between the two thyro-hyals, with which it is closely connected. The chief part of the larynx is an annular cricoid cartilage, with which two arytænoid cartilages are articulated. Membranous folds, or freely projecting cartilaginous processes of the arytænoid cartilages (Pipa), play the part of vocal ligaments. In Pipa, the larynx is extensively ossified. [...]

The Renal Organs.–The kidney is a more or less elongated organ–longer in the Urodela and Peromela, shorter in the Anura–which lies on each side of the vertebral column, its posterior end being close to, or even extending back on the dorsal side of, the cloaca. [...]

[767] The Nervous System.–The Amphibian brain is remarkable for the rudimentary condition of the cerebellum, which has the form of a mere band arching over the anterior part of the fourth ventricle. The mesencephalon is divided above, more or less distinctly, into two optic lobes. The cerebral hemispheres are always relatively large, subcylindrical in the Urodela, but wider behind than in front in the Anura, and they are generally closely united together by their inner faces. [...]

[768] The Organs of the Higher Senses.–The nasal sacs are elongated in Proteus, Menobranchus, and Siren, and not covered by nasal bones or alinasal cartilages. In the other Amphibia they are broader, and enclosed by cranial cartilages and ossifications. The olfactory mucous membrane is variously folded; and, in Rana, some of these folds are supported by ingrowths of the anterior cartilaginous wall of the nasal chamber. [...]

The Reproductive Organs.–The ovaria and testes are attached to the dorsal wall of the abdominal cavity, in the immediate vicinity of the kidneys, by the mesoarial and mesorchial folds of the peritoneum, which invest them.

The ovaria, when fully developed, become hollow, and in the Anura their internal cavities may be divided by septa.

The oviducts are long, usually more or less convoluted, tubes, which open posteriorly into the cloaca; while, anteriorly, their funnel-shaped apertures lie in the anterior part of the abdomen, sometimes, as in the Frogs, as far forward as the root of the lung. Their walls are glandular, and secrete a viscid substance which invests the ova in their passage down the oviduct.

In the male Urodela, the persistent Wolffian duct, already mentioned, occupies the position of the oviduct in the female, and the vasa efferentia, after traversing the kidney, open into it. This duct persists in Bombinator igneus and Discoglossus pictus; but in the male Anura, in general, the greater part of it is obliterated, only so much remaining as plays the part of ureter and vas deferens. In the Urodela accessory glands open into the cloaca, and in Triton there is a rudimentary copulatory papilla. Some female Urodela are provided with receptacula seminis. In the terrestrial Salamanders and in the anurous Rhinoderma Gayi the young are developed within the dilated uterine terminations of the oviducts. In Pipa the eggs are deposited on the back of the female, and the integument grows up round each, and encloses it in a cell, in which it undrgoes its development. In some tree—frogs (Nototrema and Opisthodelphys) the eggs are received into a sort of marsupial pouch formed by an up-growth of the margins of the dorsal integument, which, when complete, has a small posterior aperture. On the other hand, it is the male Alytes obstetricans which twists the strings of eggs laid by the female round his hind-legs and, thus cross-gartered, retires into seclusion until the young are ready to be hatched, when he resorts to the water in which the tadpoles are to perform their further metamorphoses.

Development of the Amphibia.–The yelk of the ovum undergoes complete division, in which respect the Amphibia agree with the Pharyngobranchii, Marsipobranchii, and Mammalia, and differ from other Vertebrata; though it must be remembered that the process of yelk division in the Ganoidei and Dipnoi is not yet known. [...]

The Distribution of the Amphibia.–Darwin has pointed out (Origin of Species, p. 350) that Amphibia are met with on no islands but New Zealand, New Caledonia, the Andaman Islands, and perhaps the Solomon Islands and the Seychelles. "This general absence of frogs, toads, and newts in so many true oceanic islands cannot be accounted for by their physical conditions; indeed, it seems that these are peculiarly fitted for those animals, for frogs have been introduced into Madeira, the Azores, and Mauritius, and have multipied so as to become a nuisance. But as these animals and their spawn are immediately killed (with the exception, so far as is known, of one Indian species) by sea-water, there would be great difficulty in their transportal across the sea, and therefore we can see why they do not exist in strictly oceanic islands."

Leaving the oceanic islands aside, the distribution of the Amphibia is world-wide, but the different groups are very remarkably localised. [...]

Geological Distribution.–No fossil Peromela are known. Anura occur in the Miocene deposits of France and Germany. The best preserved forms belong to the genera Palæobatrachus and Latonia, and occur in the schists of Œningen along with their tadpoles. They possess maxillary teeth, and present no important differences from existing Anura, except that, in Palæobatrachus, the sacral vertebra has coalesced with the two preceding vertebræ, while, in existing forms, only one of the præ-sacral vertebræ is known to become confluent with the sacral. Urodela also occurs in the same Miocene deposits. Of these the famous Andrias Scheuchzeri is very closely allied to Menopoma and Cryptobranchus, while other forms appear to be generically identical with Triton and Salamandra. The singular genus Orthophysis presents a good deal of resemblance to Proteus, but appears to have possessed no limbs.

The older Cainozoic and the upper and middle Mesozoic formations have yielded no Amphibia.[...]

[770] Ætiology of the Amphibia.–In taking a general survey of the relations of the different great divisions of the Amphibia, the most striking fact is their singular distinctness and isolation from one another. None of the Peromela present the slightest indication of an approximation towards the Anura or the Urodela. [...]

If we assume, as the fundamental similarities between the different divisions of the Amphibia lead us to do, that they have resulted from the modification of some one primitive form, the problem, at present seemingly insoluble, presents itself, whether these differences in structure and habit of the larvæ of the Urodela and Anura indicate that the caudate ancestor of the Anura was already different from the ancestor of the Urodela, or whether they result from modifications which have taken place in the larvæ of the Anura, since that group came into existence. [...]

Taxonomic Synopsis of the Amphibia. [I. The Urodela. II. The Anura. III. The Peromela. IV. The Labyrinthodonta.]

¹ Brongniart's "Essai d'ue Classification Naturelle des Reptiles" was not published in full until 1803. It appears in the volume of the Mémoires presentés à l'Institut par divers Savans for 1805.

THE HUXLEY FILE

C. Blinderman & D. Joyce
Clark University