CHAPTER I

AMPHIBIA

CHARACTERS AND DEFINITION–POSITION OF THE CLASS AMPHIBIA IN THE PHYLUM VERTEBRATA–HISTORICAL ACCOUNT OF THE CLASSIFICATION OF AMPHIBIA

A bird is known by its feathers, a Beast by its hairs, a Fish by its fins, but there is no such obvious feature which characterises the Amphibia and the Reptiles. In fact, they are neither fish, flesh, nor fowl. This ill-defined position is indicated by the want of vernacular names for these two classes, a deficiency which applies not only to the English language. All the creatures in question are backboned, creeping animals. Those which are covered with horny scales, and which from their birth breathe by lungs only, as Crocodiles, Tortoises, Lizards, and Snakes, are the Reptiles. The rest, for instance, Newts or Efts, Frogs and Toads, are the Amphibia. Their skin is mostly smooth and clammy and devoid of scales; the young are different from the adult in so far as they breathe by gills and live in the water, before they are transformed into entirely lung-breathing, terrestrial creatures. But there are many exceptions. _Proteus_ and _Siren_ the mud-eel, always retain their gills; while not a few frogs undergo their metamorphosis within the egg, and never breathe by gills. If we add the tropical limbless, burrowing Coecilians, and last, not least, the Labyrinthodonts and other fossil forms, the proper definition of the class Amphibia,–in other words, the reasons for grouping them together into one class, separated from the other backboned animals,–requires the examination of many other characters.

{4}So far as numbers of living species are concerned, the Amphibia are the least numerous of the Vertebrata. There are about 40 limbless, burrowing APODA; 100 URODELA or tailed two- or four-footed newts, and about 900 ANURA, or tailless, four-footed frogs and toads; in all some 1000 different species. Few, indeed, in comparison with the 2700 Mammals, 3500 Reptiles, nearly 8000 Fishes, and almost 10,000 Birds. But we shall see that the Amphibia have not only "had their day," having flourished in bygone ages when they divided the world, so far as Vertebrata were concerned, between themselves and the Fishes, but that they never attained a dominant position. Intermediate between the aquatic Fishes and the gradually rising terrestrial Reptiles they had to fight, so to speak, with a double front during the struggle of evolution, until by now most of them have become extinct. The rest persist literally in nooks and corners of the teeming world, and only the Frogs and Toads, the more recent branch of the Amphibian tree, have spread over the whole globe, exhibiting almost endless variations of the same narrow, much specialised plan. The greatest charm of the Anura lies in their marvellous adaptation to prevailing circumstances; and the nursing habits of some kinds read almost like fairy-tales.

CHARACTERS OF THE AMPHIBIA.[1]

1. The vertebrae are (_a_) acentrous, (_b_) pseudocentrous, or (_c_) notocentrous.

2. The skull articulates with the atlas by two condyles which are formed by the lateral occipitals. For exceptions see p. 78.

3. There is an auditory columellar apparatus, fitting into the fenestra ovalis.

4. The limbs are of the tetrapodous, pentadactyle type.

5. The red blood-corpuscles are nucleated, biconvex, and oval.

6. The heart is (_a_) divided into two atria and one ventricle, and (_b_) it has a conus provided with valves.

7. The aortic arches are strictly symmetrical.

8. Gills are present at least during some early stages of development.

9. The kidneys are provided with persistent nephrostomes.

10. Lateral sense-organs are present at least during the larval stage.

11. The vagus is the last cranial nerve.

12. The median fins, where present, are not supported by spinal skeletal rays.

13. Sternal ribs and a costal or true sternum are absent.

14. There is no paired or unpaired medio-ventral, copulatory apparatus.

15. Development takes place without amnion and allantois.

None of these characters is absolutely diagnostic, except 1 (_c_), and this applies only to the Anura and most of the Stegocephali.

{5}Numbers 1 (_b_), 1 (_c_), 2, 3, 4 and 12 separate the Amphibia from the Fishes.

Numbers 1, 6 (_b_), 7, 8, 9, 11, 13, 15 separate them from the Reptiles, Birds, and Mammals.

Number 2 separates them from the Fishes, Reptiles, and Birds.

Number 5 separates them from the Mammals.

Number 6 (_a_) separates them from the Fishes (excl. Dipnoi), Birds and Mammals.

We can, therefore, very easily define all the Amphibia, both recent and extinct, by a combination of the characters enumerated above. For instance, by the combination of numbers 2, 3 or 4 with either 7, 8, 9, 11, 13 or 15.

_Amphicondylous Anamnia_ would be an absolutely correct and all-sufficient diagnosis, but it would be of little use in the determination of adult specimens; and the tetrapodous character is of no avail for Apoda. _Amphicondylous animals without an intracranial hypoglossal nerve_ is a more practical diagnosis.

In the case of living Urodela and Anura the absence of any scales in the skin affords a more popular character; it is unfortunately not applicable to the Apoda, many of which possess dermal scales, although these are hidden in the imbricating transverse rings of the epidermis; and the frequent occurrence of typical scales of both ecto- and meso-dermal composition in many of the Stegocephali forces us to discard the scales, or rather their absence, as a diagnostic character of the class Amphibia. The same applies to the mostly soft, moist, or clammy, and very glandular nature of the skin.

THE POSITION OF THE CLASS AMPHIBIA IN THE PHYLUM VERTEBRATA.–There is no doubt that the Amphibia have sprung from fish-like ancestors, and that they in turn have given rise to the Reptilia. The Amphibia consequently hold a very important intermediate position. It was perhaps not a fortunate innovation when Huxley brigaded them with the Fishes as _Ichthyopsida_, thereby separating them more from the _Sauropsida_ (= Reptilia and Aves), than is justifiable,–perhaps more than he himself intended. The connecting-link, in any case, is formed by the Stegocephali; all the recent Orders, the Apoda, Urodela, and Anura, are far too specialised to have any claims to the direct ancestral connections. The line leading from Stegocephali to fossil Reptiles, notably to such Proreptilia as _Eryops_ and _Cricotus_, and even to the Lepospondylous Prosauria, is extremely gradual, and the steps are almost imperceptible. Naturally, {6}assuming evolution to be true, there must have lived countless creatures which were a "rudis indigestaque moles," neither Amphibia nor Reptilia, in the present intensified sense of the systematist. The same consideration applies equally to the line which leads downwards to the Fishes. But the great gulf within the Vertebrata lies between Fishes and Amphibia, between absolutely aquatic creatures with internal gills and "fins," and terrestrial, tetrapodous creatures, with lungs and fingers and toes. On the side of the fishes only the Dipnoi and the Crossopterygii come into consideration.

The piscine descent of the Amphibia is still proclaimed by the following features.–(1) The possession by the heart of a long conus arteriosus, provided with, in many cases, numerous valves, or at least (Anura) one series at the base, another at the beginning of the truncus where the arterial arches branch off; (2) the strictly symmetrical arrangement of these arches; (3) the trilocular heart is still like that of the Lung-fishes or Dipnoi; (4) the occurrence of as many as four or even five branchial skeletal arches in the larval stage; (5) the glottis is supported by cartilages which themselves are derivatives of posterior visceral arches; (6) the development of the vertebrae (Stegocephali and Urodela) from four pairs of arcualia, and the formation of the intervertebral joints by a split across the intervertebral ring of cartilage: this feature is unknown in Reptilia, but it occurs also in _Lepidosteus_, most probably also in _Polypterus_; (7) the hypoglossal still retains the character of a post-cranial or cervical spinal nerve; (8) the presence of lateral sense-organs; (9) the possession of external gills is of somewhat doubtful phylogenetic value, although such gills occur amongst fishes only in Dipnoi and Crossopterygii. It is not unlikely that in the Amphibia these organs owe their origin to entirely larval requirements, while the suctorial mouth of the larvae of the Anura and many fishes has certainly no ancestral meaning, but is a case of convergent development.

The usual diagnoses of the Amphibia contain the statement that they, or most of them, undergo a metamorphosis, or pass through a larval stage. The same applies to various fishes; while, on the other hand, the larval (not ancestral) stage has become permanent in the Proteidae and Sirenidae; and lastly, we cannot well speak of larvae in the viviparous _Salamandra atra_.

{7}THE EVOLUTION OF AN ADEQUATE CLASSIFICATION OF THE AMPHIBIA has been a long process. Even their recognition as a class, separate from, and of equal rank with that of, the Reptilia, was by no means generally accepted until comparatively recent times. A historical sketch of the laborious, often painful, striving for light, in France and Germany, then in England, and lastly in America, is not without interest.

The term _Amphibia_ was invented by Linnaeus for the third class of animals in his famous "Systema Naturae." It comprises a very queer assembly, which, even in the 13th edition (1767), stands as follows:–

1. REPTILES PEDATI, with the four "genera" _Testudo_, _Draco_, _Lacerta_, and _Rana_. _Lacerta_ includes Crocodiles, Lizards, and Newts!

2. SERPENTES APODES.

3. NANTES PINNATI. Elasmobranchs, Sturgeons, Lampreys, and various other fishes.

Laurenti, 1768, in a dissertation entitled "Specimen medicum, exhibens Synopsin Reptilium ...," uses Brisson's term, REPTILES, and divides them into:–

REPTILIA SALIENTIA, these are the Anura.

GRADIENTIA, namely the Urodela and Lizards.

SERPENTIA, the Snakes and the Apoda.

Brongniart, 1800, "Essay d'une classification naturelle des Reptiles,"[2] distinguishes:–

CHELONII, SAURII, OPHIDII, BATRACHII; the last for the Frogs, Toads, and Newts.

Latreille, 1804, "Nouveau Dict. Hist. Nat." xxiv.,[3] accepts the four Orders of Brongniart's "Reptiles," but clearly separates the fourth Order, "BATRACHII," from the rest by the following, now time-honoured, diagnosis: _Doigts des pattes n'ayant pas d'ongles; des branchies, du moins pendant un temps; des métamorphoses_. But there is not one word about "Amphibia" in opposition to "Reptilia."

Duméril, 1806, "Zoologie analytique" (p. 90), and "Élémens de l'histoire naturelle," 1807, divides the "Reptiles batraciens," or "Batracii," into ECAUDATI and CAUDATI; he also introduces the terms "ANOURES" and "URODÈLES" as their equivalents; but since these terms appear in the French form purists do not admit their having any claim to recognition!

Oppel, 1811, "Die Ordnungen, Familien und Gattungen der Reptilien," establishes the term APODA for the Coeciliae, and recognises their affinity to the Ecaudata and Caudata by removing them from the Snakes.

De Blainville, 1816, "Prodrome d'une nouvelle distribution du règne animal"[4]–

AMPHIBIENS SQUAMIFÈRES. [The Reptilia.]

" NUDIPELLIFÈRES s. Ichthyoides. [The Amphibia.]

{8}Merrem, 1820, "Tentamen systematis Amphibiorum."

PHOLIDOTA. [The Reptilia.]

BATRACHIA: _APODA_. _SALIENTIA_. { Mutabilia [with metamorphosis, _e.g._ _GRADIENTIA_ { Newts.] { Amphipneusta [Perennibranchiate Urodeles.]

F. S. Leuckart, 1821, "Einiges ueber die fischartigen Amphibien."[5]

MONOPNOA. [The Reptilia.]

{ with temporary gills: Ecaudata + Caudata pt. DIPNOA. [The Amphibia] { with permanent gills: "Proteidae," _Menopoma_ { and _Amphiuma_.

Latreille, 1825, "Familles naturelles du règne animal." The Vertebrata are divided into _Haematherma_ and _Haemacryma_. These terms for warm and cold-blooded creatures were later on amended by Owen to _Haematotherma_ and _Haematocrya_. The latter are divided by Latreille as follows:–

REPTILIA. Still including the Coeciliae amongst the Snakes.

AMPHIBIA { Caducibranchiata. { Perennibranchiata.

PISCES.

Wagler, 1830, "Systema Amphibiorum."

TESTUDINES, CROCODILI, LACERTAE, SERPENTES, ANGUES, COECILIAE, RANAE, ICHTHYODI.

RANAE I. _AGLOSSA._

" II. _PHANEROGLOSSA_: 1. Cauda nulla. [The Anura.]

" " 2. Cauda distincta. [The Salamandridae.]

ICHTHYODI I. _ABRANCHIALES._ _Menopoma_ [_Cryptobranchus_] and _Amphiuma_.

" II. _BRANCHIALES._ [The Perennibranchiate Urodela.]

J. Müller, 1831, "Beiträge zur Anatomie ... der Amphibien."[6]

GYMNOPHIONA, DEROTREMATA, PROTEIDAE, SALAMANDRINA, BATRACHIA.

J. Bell, 1836, Todd's "Cyclopaedia of Anatomy and Physiology," Art. "Amphibia."

AMPHIPNEUSTA, the Perennibranchiate Urodeles; ANOURA, URODELA;

ABRANCHIA, _Menopoma_ and _Amphiuma_; APODA.

Stannius, 1856, "Handbuch der Zootomie: Anatomie der Wirbelthiere." (2nd ed.)

AMPHIBIA MONOPNOA. The Reptilia.

AMPHIBIA DIPNOA. 1. URODELA. _PERENNIBRANCHIATA._ _DEROTREMATA_: _Amphiuma_ and _Menopoma_. _MYCTODERA._[7] {9} 2. BATRACHIA. _AGLOSSA._ _PHANEROGLOSSA_: Systomata = Engystomatidae. Bufoninae. Without manubrium sterni. Raninae. With manubrium. Hyloidea. With adhesive finger-discs. _GYMNOPHIONA._

Gegenbaur, 1859, "Grundzüge der vergleichenden Anatomie."

AMPHIBIA as a separate class, equivalent to that of the REPTILIA, are divided into the four Orders: _PERENNIBRANCHIATA_, _SALAMANDRINA_, _BATRACHIA_, and _GYMNOPHIONA_. In the second edition of the "Grundzüge" (1870) they are divided into _URODELA_, _ANURA_, and _GYMNOPHIONA_.

Huxley, 1864, "The Elements of Comparative Anatomy."

MAMMALS. SAUROIDS, subsequently changed into SAUROPSIDA = Reptilia + Aves. ICHTHYOIDS, " " ICHTHYOPSIDA = Amphibia + Pisces.

Haeckel, 1866, "Generelle Morphologie."

Amphibia. A. PHRACTAMPHIBIA s. Ganocephala = Labyrinthodonta + Peromela [Apoda]. B. LISSAMPHIBIA s. Sozobranchia = Sozura [Urodela] + Anura.

Cope, 1869.[8]

STEGOCEPHALI, GYMNOPHIDIA, URODELA, PROTEIDEA, TRACHYSTOMATA, ANURA.

Huxley, 1871, "A Manual of the Anatomy of Vertebrated Animals."

Amphibia I. SAUROBATRACHIA [v.d. Hoeven's term] s. URODELA 1. Proteidea. 2. Salamandridae. II. LABYRINTHODONTA. III. GYMNOPHIONA. IV. BATRACHIA s. ANURA.

Boulenger, 1882, "Catalogue of the BATRACHIA GRADIENTIA s. CAUDATA and BATRACHIA APODA," divides the Caudata simply into: _SALAMANDRIDAE_, _AMPHIUMIDAE_, _PROTEIDAE_, and _SIRENIDAE_.

1882, "Cat. Batrachia Salientia s. Ecaudata," see p. 140.

Cope, 1890, "Synopsis of the Families of Vertebrata."[9]

CLASS BATRACHIA. Sub-Class I. STEGOCEPHALI. Order 1. Ganocephali: _Trimerorhachis_, _Archegosaurus_. 2. Rhachitomi: _Eryops_. 3. Embolomeri: _Cricotus_. 4. Microsauri: _Branchiosaurus_, _Hylonomus_, etc. {10} Sub-Class II. URODELA. Order 1. Proteidae: _Proteus_. 2. Pseudosauria. [All the rest of the Urodela + Coeciliidae.] 3. Trachystomata: Sirenidae. Sub-Class III. SALIENTIA.

P. and F. Sarasin, 1890, "Zur Entwicklungsgeschichte der Ceylonesischen Blindwühle, Ichthyophis glutinosa."[10]

Sub-Class I. ARCHAEOBATRACHI s. STEGOCEPHALI. II. NEOBATRACHI. Order 1. _URODELA._ _a_. Salamandroidea. [The Urodela.] _b_. Coeciloidea = Amphiumidae + Coeciliidae. 2. _ANURA._

The classification adopted in this volume is as follows:–

CLASS AMPHIBIA. Sub-Class I. Phractamphibia. Order I. Stegocephali Lepospondyli. Sub-order 1. Branchiosauri. Sub-order 2. Aistopodes. Order II. Stegocephali Temnospondyli. Order III. Stegocephali Stereospondyli. Sub-Class II. Lissamphibia. Order I. Apoda. Order II. Urodela. Order III. Anura. Sub-order 1. Aglossa. Sub-order 2. Phaneroglossa.

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