University of Virginia Library

3. CHAPTER III.

To prosecute successfully our inquiry "What is a Frog?" it will be well now to make acquaintance with the more remarkable forms contained in its Order, after which, by considering the other Batrachian orders, we may arrive at a certain appreciation of its Class.

The Frog's own genus (Rana), which contains about 40 species, has its head-quarters in the East Indies and in Africa, but extends over all the great regions of the world, except Australia and parts more southerly still and except countries situate above 66 ° north latitude. In South America, however, but a single species is as yet known to exist.

Amongst the largest species are Rana tigrina, of India, and the Indian Archipelago, and the bull-frog ( R. Mugiens) of North America. The latter animal may often be seen in the Gardens of the Zoological Society, where it is fed on small birds—a sparrow being easily engulphed within its capacious jaws.

The Edible Frog, par excellence (R. esculenta), is

[Description: Image of page 22. ]

found in England as well as on the Continent of Europe. It is as widely distributed over the Old World as is R. temporaria, but it is unknown in America. It is easily to be discriminated from the common species by the absence of that dark, sub-triangular patch which extends backwards from the eye in R. temporaria.

The male of R. esculenta is further to be distinguished from the male of the common Frog by the fact of its having the floor of the mouth, on each side, distensible as a pouch—the pouches, when distended, standing out on each side of the head. These pouches are called "vocal sacs," and no doubt aid in intensifying these animals' croak, which is so powerful that (on account of it and because of the country where they are common) they have been nicknamed "Cambridgeshire Nightingales." Specimens

[Description: Image of page 23. ]

from Cambridgeshire are preserved in the British Museum.

A large South American Frog ( Ceratophrys cornuta),

[Description: Image of page 24. ]

which devours other smaller Frogs as well as small birds and beasts, is noteworthy on account of the singular bony plates which are enclosed in the skin of its back: a character which it shares with a small South American Toad ( Brachycephalus ephippium), and which we shall hereafter see to be a point of special interest.

A Frog newly discovered [8] (of a new genus but allied to Rana), called Clinotarsus, [9] has been represented, in the hope that by the wider circulation of a figure of it, it may be recognized, and its habitat so ascertained (Fg. 5).

The common Toad (Bufo vulgaris) is as widely distributed over the earth's surface as is Rana esculenta. It is less aquatic than the Frog, and more sluggish in its motions. In shape it resembles the Frog, but is more swollen, with much shorter legs and a warty skin. The toes are less webbed, and the margin of the upper jaw, as well as the lower, is entirely destitute of teeth. The jaws are similarly toothless in all toads.

The toad is provided with an oblong, elongated gland called (Parotoid) behind each eye. These glands emit a milky secretion which is acrid and very unpleasant to the mouth of some carnivorous animals. Those who have observed a dog attacking a toad can hardly have failed to notice the disgust

[Description: Image of page 25. ]

which the former animal seems to exhibit by the copious flow of its saliva, its. many head-shakings, &c. The toad's secretion, however, cannot be said to be poisonous, and certainly it is not so in the mode in which the venom of serpents is poisonous, since a chicken may be inoculated with it, and yet appear to suffer no injury whatever beyond the infliction of the slight wound necessary for the performance of the operation. Nevertheless the secretion exercises a very decided effect upon certain animals, since the tadpoles both of frogs and of salamanders are very powerfully affected by being kept in the same water with a toad, if the latter be specially irritated in order to make it discharge its pungent and irritating secretion.

True poison and organs fitted both to inflict wounds and to convey the venom into them are not indeed found in any animals which are even near allies of

[Description: Image of page 26. ]

the frogs and toads. Nevertheless a very perfect organ for both wounding and poisoning has been discovered by Dr. Günther to exist in a certain fish ( Thalassophryne reticulata), belonging to a group which, on account of their superficial resemblances to frogs, are termed "Batrachoid."

He found in the fish no less than four spines, each perforated like the tooth of a viper, and each having a sac at its base. One such poison-spine was situated on each side of the hinder part of the head in front

[Description: Image of page 27. ]

of the gill-opening. Two others were dorsal spines placed one behind the other on the mid-line of the back. These poison-organs are probably only used for defence. They are formed, however, on the very same type as are the poison fangs of vipers. Unlike the latter, however, they are not modified teeth, nor are they situated within the mouth as they always are in poisonous serpents.

A Frog (Pelobates fuscus) which is common in France (and which is interesting on account of the form of its skull hereafter to be pointed out), though really harmless enough, has a singular power of making itself offensive.

Both males and females of this species utter a kind of croak, and both, if the thigh is pinched, produce a sound like the mewing of a cat. At the same time they emit a strong odour, which is like that of garlic, and becomes stronger as the animals are more disturbed. This emission not only affects the sense of

[Description: Image of page 28. ]

smell, but even makes the eyes water as mustard or horseradish does.

This singular power, together with the acrid secretion of the toad, are the nearest approximation to venomous properties possessed by any members of the order, no toad—not even the giant of the order Bufa agua—being really poisonous.

A small Frog, by no means uncommon in France and Germany (Alytes obstetricans), has a very singular habit. The female lays its eggs (about sixty in number) in a long chain, the ova adhering successively to one another by their tenacious investment. The male twines this long chaplet round his thighs, so that he acquires the appearance of a courtier of the time of James I. arrayed in trunk hose and puffed breeches. Thus encumbered, he retires into some burrow (at least during the day) till the period when the young are ripe for quitting the egg. Then he seeks water, into which he has not plunged many minutes when the young burst forth and swim away, and he, having disencumbered himself of the remains of the ova, resumes his normal appearance.

Certain Frogs (forming a very large group) are termed Tree-Frogs, from their adaptation to arboreal life by means of the dilatation of the ends of the digits into sucking discs, by which they can adhere to leaves. One of them, the common green Tree-Frog (Hyla arborea), is spread over Europe, Asia, and Africa, in the same manner as R. esculenta except that it is not found in the British Isles. A few toads also have the tips 6f their digits similarly dilated. Such, e.g., is the case with the

[Description: Image of page 29. ]

genera Kaloula of India, and Brachymerus of South Africa.

The female of a peculiar American Tree-Frog ( Nototrema marsupiatum) has a pouch extending over the whole of the back and opening posteriorly.

Into this the eggs are introduced for shelter and protection. A dorsal pouch also exists in the allied American genus, Opisthodelphys. An American species of Hylodes has the habit of laying its eggs in trees singly in the axils of leaves, and the only water they can obtain is the drop or two which may from time to time be there retained.

A still more remarkable mode of protecting the egg is developed by the Great Toad of tropical America (Pipa americana). In this case the skin

[Description: Image of page 30. ]

of the female's back at the laying season thickens greatly and becomes of quite a soft and loose texture. The male, as soon as the eggs are laid, takes them and imbeds them in this thick, soft skin, which closes over them. Each egg then undergoes its process of development so enclosed, and the tadpole stage is, in this animal, passed within the egg, so that the young toads emerge from the dorsal cells of the mother completely developed miniatures of the adult. As many as 120 of these dorsal cells have been counted on the back of a single individual.

The only instance of a similar cutaneous modification is that pointed out by Dr. Günther [10] in the

[Description: Image of page 31. ]

skin of the belly of the Siluroid fish, Aspredo batrachus. Here he found that " the whole lower surface of the belly, thorax, throat, and even a portion of the pectoral fins, showed numerous shallow, round impressions, to which a part of the ova still adhered." He concludes that "it is more than probable that towards the spawning time the skin of the lower parts becomes spongy, and that, after having deposited the eggs, the female attaches them to it by merely lying over them." "When the eggs are hatched the excrescences disappear, and the skin of the belly becomes smooth as before." Even in the highest class of animals ( Mammalia) we are familiar, in the Kangaroo and Opossum order ( Marsupialia), with a special external receptacle (the marsupial pouch) for the protection and secure development of the

[Description: Image of page 32. ]

young; but nothing of the kind exists amongst birds or reptiles. In fishes, however, the male of the little Sea-horse (Hippocampus) is provided with a ventral pouch in which the eggs are sheltered, and the same class presents us with a mode of carrying the eggs still more bizarre than that of Alytes obstetricans just related. In the fish Arius fissus the male actually carries about the ova in the mouth, protected by the jaws, till relieved of the inconvenience by the hatching of the young fry.

A South African Toad ( Dactylethra capensis) is interesting, as we shall hereafter see, on account of certain anatomical points in which it agrees with Pipa, and differs from all other Anoura. No interesting facts, however, are known as to its habits.

Another noteworthy form is the Mexican Rhinophrynus dorsalis, the exceptional characters of which are the tongue, which is free in front instead of behind, and the enormous Spur-like tarsal tubercle.

Almost all frogs and toads pass the first stages of their existence in water, going through a free,

[Description: Image of page 33. ]

tadpole stage, and all are more or less aquatic when adult. The only exceptions are Pipa,Nototrema , Opisthodelphys, and the Hylodes before referred to. Very many kinds, however, are, when adult, inhabitants of trees. The question may suggest itself to some, "Are there any, which can be said in any sense to be aërial animals?" Birds are almost all

[Description: Image of page 34. ]

capable of true flight, as also are those aërial existing beasts the Bats, and as were those extinct reptiles the Pterodactyles. Certain squirrels and opossums can take flitting jumps by means of an extension of the skin of the flank, and a similar, though much greater extension, supported by elongated freely ending ribs, is found in the little lizards ( Draco) called Flying Dragons.

The class of Fishes supplies us, also, with an example of aërial locomotion in the well-known Flying Fish.

Since, then, every other class of vertebrate animals (Beasts, Birds, Reptiles and Fishes) presents us with more or fewer examples of the aërial species, we might perhaps expect that the Frog class would also exhibit some forms fitted for progression through the air. We cannot say with certainty that such is the case; but Mr. Alfred Wallace, in his travels in the Malay Archipelago, encountered in Borneo a Tree-frog (Rhacophorus) to which he considers the term "flying" may fairly be applied, and of which he says, it "is the first instance known of a flying frog." Of this animal he gives us the following account:—

"One of the most curious and interesting creatures which I met with in Borneo was a large tree-frog which was brought me by one of the Chinese workmen. He assured me that he had seen it come down, in a slanting direction, from a high tree as if it flew. On examining it I found the toes very long and fully webbed to their extremity, so that, when expanded, they offered a surface much larger than

[Description: Image of page 35. ]

the body. The fore-legs were also bordered by a membrane, and the body was capable of considerable inflation. The back and limbs were of a very deep shining green colour, the under surface and the inner toes yellow, while the webs were black rayed with yellow. The body was about four inches long, while the webs of each hind foot, when fully expanded, covered a surface of four square inches, and the webs of all the feet together about twelve square

[Description: Image of page 36. ]

inches. As the extremities of the toes have dilated discs for adhesion, showing the creature to be a true Tree-frog, it is difficult to imagine that this immense membrane of the toes can be for the purpose of swimming only, and the account of the Chinaman that it flew down from the tree becomes more credible."

The great group of Frogs and Toads, rich as it is in genera and species, and widely as it is diffused over the earth's surface, is one of singular uniformity of structure. The forms most aberrant from our type, the common Frog, have now been noticed, except that perhaps the maximum respectively of obesity and slenderness may be referred to. In the former respect the Indian Toad, Glyphoglossus, may serve as an example, and for the latter may be selected Hylorana jerboa.

FOOTNOTES: Chapter 3