CHAPTER XXIX.

AMPHIBIA (Frogs and Toads).

"To any person," observes the eloquent historian of British Reptiles, "capable of appreciating the interest attached to the study of physiological phenomena, the contemplation of an animal which at one period of its life is endowed exclusively with the organs of aquatic respiration, resembling the gills of fishes, with means of locomotion adapted only to a constant residence in the water, and with a digestive apparatus fitted exclusively for th assimilation of vegetable food, assuming by degrees the function of atmospheric respiration, acquiring limbs which are formed for leaping on land with great strength and agility, and manifesting the most voracious carnivorous appetite, will not only excite feelings of the deepest admiration, but necessarily lead to the investigation of the laws by which such extraordinary changes are governed, and of the relations which they bear to the theory of continuous affinity, and to that of progressive development through the whole of the animal kingdom.”·

Such phenomena are exhibited by the Toads, Frogs, and Newts, the familiar representatives of that limited

* Bell's "Brit. Rept.," p. 72.

Class of animals whose scientific appellation we have inscribed at the head of this chapter. They thus afford a beautiful link in that tissue of "chain-mail" which constitutes the Plan of Nature; for they evidently hold an intermediate position between the FISHES, whose respiration is exclusively aquatic, and the true REPTILES, in which this vital function is exclusively aerial.

Let us look a little more closely at this curious point,— the metamorphosis which the AMPHIBIA undergo, and the accompanying change in the character of their breathing organs. In the month of April, in almost every ditch and pool in the country, we see large masses of clear jelly, with black dots distributed at even distances throughout, or long strings of the same substance, in which the black dots are arranged in a double row. The former is the spawn of the Frog, the latter that of the Toad; and each dot is the maturing embryo of a single egg, which latter is a clear globe of about one-fifth of an inch in diameter. When the spawn is laid, the embryo is an opaque globule, darker at one side than at the other. In a few days, however, this begins to take the form of an animal-the head, the body, and the tail being distinct, as the little creature lies on its side within the egg, coiled up in a semicircle. Soon a kind of wart buds from each side of the neck-the future gills; and currents of water are seen to stream to and from these important, but as yet minute, organs.

As time passes, the gills divide into branches, the nostrils and the eyes appear, and traces of the mouth may be discerned. Meanwhile, the power of voluntary movement, at first confined to the head and tail, increases; and the

little prisoner, as if impatient of confinement, tries to straighten itself by spasmodic efforts, and at length succeeds in rupturing the skin of the egg and becomes free.

It is now a Tadpole-a fish-like creature, without limbs, with an enormous head, and a body thinned off to

Transformations of Frog.

a long tail, which is furnished with a broad finny expansion above and below. The gills now attain their greatest size, and consist on each side of a pair of much-branched tufts, which, under the microscope, present a most inter

esting spectacle. The blood, forced from the heart in regular pulsations, is seen to diverge into each of the main gill-stems, sending off lateral streams to every tiny branchlet the red globules are seen to chase each other along the tortuous vessels, to pass to the extremity of every ramification, and then, turning, follow a backward course, until they reach the heart, the fountain from whence they issued.

But now these organs begin to disappear: they gradually diminish, until at length they can no longer be discerned externally, though their function is carried on in a cavity of the body on each side. The little animal increases rapidly in size, but does not change this its fish-like form for a considerable period, though minor modifications may be traced. Thus the mouth becomes developed, the eyes are perfectly formed, and the tail-fin grows greatly in perpendicular breadth, and is a powerful organ of locomotion. The little Tadpole begins greedily to devour vegetable matter, and manifests the effect of this diet in the change of its own hue from a dull black to a soft olivegreen, with golden specks on the under parts.

At length the period approaches when the Tadpole must leave its aquatic life, and become terrestrial;—at least it must cease to respire water, and must derive its vitality from the air. The first step to such a change, is the development of limbs. First appear the hind-legs, in the form of a pair of minute budding warts, which lengthen, become bent, and shoot forth tiny toes at the extremity. The fore-legs, always a little later than the hind, accompany the latter in their gradual progress. As the new organs of motion are acquired, the old one-the vibrating

tail with its fin-is lost. It is not thrown off, but its substance is gradually absorbed into the body. As this process takes place during the growth of the legs, when it is completed, the Tadpole has become a little Frog. The minute orifices which admitted the water into the gill-chamber have, at the same time, become closed, and breathing is henceforth performed exclusively by means of lungs, which are capacious sacs, subdivided internally into large cells.

Such, then, is the metamorphosis which obtains in the most elevated forms of this Class, as our common Frog and Toad; and it may be witnessed with slight precautions by any one who will take the trouble to collect a mass of spawn from the nearest ditch, and transfer it to a fresh-water aquarium. In the Newts, which are no less common, the metamorphosis is less complete, and we perceive in their ultimate condition a closer alliance with FISHES; since their limbs are small and feeble, their broad finny tail is retained through life, and is the principal organ of locomotion; for, in general, they continue more exclusively aquatic in their mode of life than the adult Frog or Toad.

The eggs of the Newts are not deposited in a mass, but singly, and that under interesting conditions. Professor Bell thus describes the process in the case of our largest and finest species, the Common Warty-newt (Triton cristatus), the males of which are conspicuous enough in the vessels of the dealers in aquatic animals, in Covent Garden Market and elsewhere, by their roughened blackish upper parts, their high notched back-fin, and their rich orange under-parts, spotted with black. The female is of less brilliant hues, and is destitute of the tail fin.