of the male, usually the fore limbs (pleurodele newt, Ecaudata), rarely the hind limbs (a few American and European newts), according to the mode of amplexation, acquire a greater development, and are often armed with temporary horny excrescences which drop off after the pairing season. These asperities usually form brush-like patches on the inner side of one or more of the digits, but may extend over the inner surface of the limbs and on the breast and chin; the use of them on these parts is sufficiently obvious, but they are sometimes also present, without apparent function, on various parts of the foot, as in Discoglossus, Bombinator, and Pelodytes. In some species of the South American frogs of the genus Leptodactylus the breast and hands are armed with very large spines, which inflict deep wounds on the female held in embrace.
In most of the Caudata, the eggs are deposited singly in the axils of water plants or on leaves which the female folds over the egg with her hind limbs. The eggs are also deposited singly in some of the lower Ecaudata. In many of the Ecaudata, and in a few of the Caudata and Apoda, the eggs are laid in strings or bands which are twined round aquatic plants or carried by the parent; whilst in other Ecaudata they form large masses which either float on the surface of the water or sink to the bottom.
A few batrachians retain the ova within the oviducts until the young have undergone part or the whole of the metamorphosis. Viviparous parturition is known among the Caudata (Salamandra, Spelerpes fuscus), and the Apoda (Dermophis thomensis, Typhlonectes compressicauda); also in a little toad (Pseudophryne vivipara) recently discovered in German East Africa (41).
Development and Metamorphosis.—In a great number of batrachians, including most of the European species, the egg is small and the food-yolk is in insufficient quantity to form an external appendage of the embryo. But in a few European and North American species, and in a great many inhabitants of the tropics, the egg is large and a considerable portion of it persists for a long time as a yolk-sac. Although the segmentation is always complete, it is very irregular in these types, some of which make a distinct approach to the meroblastic egg.
With the exception of a number of forms in which the whole development takes place within the egg or in the body of the mother, batrachians undergo metamorphoses, the young passing through a free-swimming, gill-breathing period of considerable duration, during which their appearance, structure, and often their régime, are essentially different from those of the mature form. Even the fossil Stegocephalia underwent metamorphosis, as we know from various larval remains first described as Branchiosaurus. They are less marked or more gradual in the Apoda and Caudata than in Ecaudata, in which the stage known as tadpole is very unlike the frog or toad into which it rather suddenly passes (see Tadpole). In the Caudata, external gills (three on each side) persist until the close of the metamorphosis, whilst in the Apoda and Ecaudata they exist only during the earlier periods, being afterwards replaced by internal gills.
Many cases are known in which the young batrachian enters the world in the perfect condition, as in the black salamander of the Alps (Salamandra atra), the cave salamander (Spelerpes fuscus), the caecinan Typhlonectes, and a number of frogs, such as Pipa, Rhinoderma, Hylodes, some Nototrema, Rana opisthodon, &c. A fairly complete bibliographical index to these cases and the most remarkable instances of parental care in tailless batrachians will be found in the interesting articles by Lilian V. Sampson (42), and by G. Brandes and W. Schoenichen (43). It will suffice to indicate here in a synoptic form, as was done by the present writer many years ago, when our knowledge of these wonders of batrachian life was far less advanced than it is now, the principal modes of protection which are resorted to:—
1. Protection by means of nests or nurseries.
A. In enclosures in the water.—Hylafaber.
B. In nests in holes near the water.—Rhacophorus, Leptodactylus.
C. In nests overhanging the water.—Rhacophorus, Chiromantis, Phyllpmedusa.
D. On trees or in moss away from the water.—Rana opisthodon, Hylodes, Hylelia platycephala.
E. In a gelatinous bag in the water.—Phrynixalus, Salamandrella.
2. Direct nursing by the parents.
A Tadpoles transported from one place to another.—Dendrebates, Phyllobates, Sooglossus.
B. Eggs protected by the parents who coil themselves round or “sit” on them.—Mantophryne, Desmognathus, Autodax, Plethodon, Cryptobranchus, Amphiuma, Ichthyophis, Hypogeophis, Siphonops.
C. Eggs carried by the parents.
(a) Round the legs, by the male.—Alytes.
(b) On the back, by the female.
(1) Exposed.—Hyla goeldii, H. evansii, Ceratohyla.
(2) In cell-like pouches.—Pipa.
(3) In a common pouch.—Nototrema, Amphignathodon.
(c) On the belly.
(1) Exposed, by the female.—Rhacophorus reticulatus.
(2) In a pouch (the produced vocal sac), by the male.—Rhinoderma.
(d) In the mouth, by the female.—Hylambates brevirostris.
Geographical Distribution.—If a division of the world according to its batrachian faunae were to be attempted, it would differ very considerably from that which would answer for the principal groups of reptiles, the lizards especially. We should have four great realms:—(1) Europe and Northern and Temperate Asia, Africa north of the Sahara (palaearctic region) and North and Central America (nearctic region); (2) Africa and South-Eastern Asia (Ethiopian and Indian region); (3) South America (neotropical region); and (4) Australia (Australian region). The first would be characterized by the Caudata, which are almost confined to it (although a few species penetrate into the Indian and neotropical regions), the Discoglossidae, mostly Europaeo-Asiatic, but one genus in California, and the numerous Pelobatidae; the second by the presence of Apoda, the prevalence of firmisternal Ecaudata and the absence of Hylidae; the third by the presence of Apoda, the prevalence of arciferous Ecaudata and the scarcity of Ranidae, the fourth by the prevalence of arciferous Ecaudata and the absence of Ranidae, as well as by the absence of either Caudata or Apoda. Madagascar might almost stand as a fifth division of the world, characterized by the total absence of Caudata, Apoda, and arciferous Ecaudata. But the close relation of its very rich frog-fauna to that of the Ethiopian and Indian regions speaks against attaching too great importance to these negative features. It may be noted here that no two parts of the world differ so considerably in their Ecaudata as do Madagascar and Australia, the former having only Firmisternia, the latter only Arcifera. Although there is much similarity between the Apoda of Africa and of South America, one genus being even common to both parts of the world, the frogs are extremely different, apart from the numerous representatives of the widely distributed genus Bufo. It may be said that, on the whole, the distribution of the batrachians agrees to some extent with that of fresh-water fishes, except for the much less marked affinity between South America and Africa, although even among the former we have the striking example of the distribution of the very natural group of the aglossal batrachians, represented by Pipa in South America and by Xenopus and Hymenochirus in Africa.
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