This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1866). The most remarkable examples of the Caducibranchtate Amphibia are the Frogs, the Toads, and the Newts, so common in our own country; and the metamorphosis of these creatures from the tadpole- or fish-condition under which they leave the egg, to their perfect air-breathing and four-footed state, is a matter of common observation. We select the Newt (Triton cristatus) as an example of the changes which these amphibians undergo as they advance towards maturity.
(1867). Immediately before leaving the egg, the tadpole of the Salamander, or Water-Newt (fig. 329, a), presents both the outward form and internal structure of a fish. The flattened and vertical tail fringed with a broad dorsal and anal fin, the shape of the body, and the gills appended to the sides of the neck are all apparent; so that, were the creature to preserve this form throughout its life, the naturalist would scarcely hesitate in classing it with fishes properly so called.
Fig. 329. Larvae of Triton.
(1868). When first hatched (fig. 329, b)*, it presents the same fishlike body, and rows itself through the water by the lateral movements of the caudal fin. The only appearance of legs as yet visible consists in two minute tubercles, which seem to be sprouting out from the skin immediately behind the branchial tufts, and which are, in fact, the first buddings of anterior extremities. Nevertheless, to compensate to a certain extent for this total want of those prehensile limbs which afterwards become developed, two supernumerary organs are provisionally furnished, in the shape of two minute claspers, seen in the figure, situated on each side of the mouth; by means of these the little being holds on to the subaquatic leaves, and thus prevents itself from being washed away by the slightest current.
(1869). Twelve days after issuing from the egg, the two fore legs, which at first resembled two little nipples, have become much elongated, and are divided at their extremity into two or three rudiments of fingers (fig. 329, c.) The eyes, which were before scarcely visible, and covered by a membrane, distinctly appear. The branchiae, at first simple, are divided into fringes, wherein red blood now circulates; the mouth has grown very large; and the whole body is so transparent as to reveal the position of the viscera within. Its activity is likewise much increased: it swims with rapidity, and darts upon minute aquatic insects, which it seizes and devours.
Fig. 330. Larrse of Triton.
(1870). About the twenty-second day (fig. 330, d) the Tadpole, for the first time, begins to emit air from its mouth, showing that the lungs have begun to be developed. The branchiae are still large. The fingers upon the fore-legs are completely formed; the hind-legs begin to sprout beneath the skin; and the creature presents, in a transitory-condition, the same external form as that which the Siren lacertina permanently exhibits.
* Vide Rusconi, Amours des Salamandres aquatiques, et Developpement duTetard de ces Salamandres depuis l'oeuf jusqu'a l'animal parfait. 4to. Milan, 1821.
(1871). By the thirty-sixth day the young Salamander (fig. 330, e) has arrived at the development of the Proteus anguinus; its hind-legs are nearly completed, its lungs have become half as long as the trunk of its body, and its branchiae more complicated in structure.
(1872). At about the forty-second day the Tadpole begins to assume the form of an adult Triton (fig. 330, f): the whole body becomes shorter; the fringes of the branchiae are rapidly obliterated, so that in five days they are reduced to simple prominences covered by the skin of the head; and the gill-openings at the sides of the neck, which, as in fishes, allowed the water to escape from the mouth, and were in like manner covered with an operculum formed by a fold of the integument, are gradually closed; the membranous fin of the tail contracts, the skin becomes thicker and more deeply coloured, and the creature ultimately assumes the form and habits of the perfect Newt, no longer possessing branchiae at all, but breathing air, and in every particular completely converted into a reptile.
(1873). But however curious the phenomena attending the development of the tadpoles of the amphibious Reptiles may be to the observer who merely watches the changes perceptible from day to day in their external form, they acquire a tenfold interest to the physiologist who traces the progressive evolution of their internal viscera - more especially when he finds that in these creatures he has an opportunity afforded him of contemplating (displayed before his eyes, as it were, upon an enlarged scale) those phases of development through which the embryo of every air-breathing vertebrate animal must pass while concealed within the egg. The division, therefore, of Reptiles into such as undergo a metamorphosis, and such as do not, is by no means philosophical, although convenient to the zoologist: all Reptiles undergo a metamorphosis, though not to the same extent. In the PEKENNiBRAisr-chiata the change from the aquatic to the air-breathing animal is never fully completed; in the Caducibranchiata the change is accomplished after the embryo has escaped from the ovum; and in the Reptilia proper, as well as in Birds and Mammals, which are generally said to undergo no metamorphosis, the changes referred to are accomplished in ovo during the earliest periods of the formation of the foetus.
(1874). The second order of Reptiles (Ophidia) includes the Serpent tribes, animals entirely deprived of external locomotive extremities, and nevertheless endowed with attributes at once formidable and surprising. Absolutely without limbs or any apparent means of progression, the scale-clad serpent makes its way in either element with equal facility, and walks or leaps, or climbs or swims, at will. Destitute of any prehensile members, it seizes and devours the strongest and most active prey: it binds its victim in a living rope; or, with a single scratch inflicted by its venomed fangs, speedily destroys the stoutest assailant.