SUB-CLASS II. AMPHIBIA.
(Naked Reptiles.)
So important are the differences between the animals we have been considering, and those at which we are now arrived, that some zoologists of the greatest eminence consider them as forming two distinct Classes, or groups equal in rank to the Mammalia, Birds, and Fishes; though others of high reputation prefer to give to the latter no higher rank than that of an Order of the Class Reptilia, thus making this group only equal in value to the Testudinata, or Sauria respectively. It is principally for the convenience of describing both in a single volume, that we assign to the Amphibia an intermediate place; making them a group of equal rank to the rest of the Reptilia put together, but not one of the primary divisions of the Vertebrata.
The Amphibia, though widely distributed, are not very numerous, nor do any of them attain a large size. Destitute for the most part of the attractions of form and colouring, they are not generally considered interesting, except to the philosophical observer of the works of God; but he, however, perceives in them abundant evidences of creative wisdom.
The most obvious character of these animals is that of their integuments. The skin is naked, not being defended by plates or scales, or any similar apparatus; it is in general moist or clammy, and in many cases is provided with numerous glands which pour out a viscid secretion. The outer skin, or epidermis, is sloughed at irregular intervals like that of the Lizards and Serpents. The form is generally more or less lizard-like, but in one Order, that of the Frogs, this form is lost at an early period of life. They possess, in general, four limbs; but, like the Lizards, they shew a tendency to merge into a serpentiform condition, by the lengthening of the body, and loss of the hinder extremities in the Sirens, and by the total absence of feet, and snake-like contour of the Cæciliæ.
Most of the Amphibia undergo a metamorphosis, or change of form, which is connected with a change in the character of their respiration and in the medium in which they live. Thus the Frog is, in its first stage of active existence, a fish-like Tadpole, breathing the water by means of gills, but afterwards undergoes a great change of form, gradually acquires four well-developed limbs, throws off its tail, and at the same time loses its external gills, and becomes an air-breathing animal, possessed of internal lungs, and capable of crawling on the land as well as swimming in water. Some of the genera, indeed, possess both series of respiratory organs during their whole life, and are consequently Amphihia in a more strict and literal sense, possessing a capacity of living and breathing in two elements.
The reproduction is effected by means of eggs, which are numerous, more or less globular in figure, pellucid, and destitute of a calcareous covering. In general, they are enveloped in a transparent glaire or jelly.
The organization of the skeleton, as well as other peculiarities in the anatomy, manifest that the Amphibia are formed on a grade of structural development, inferior to that of the Enoplia. The skull is united to the spine by two processes,
SKELETON OF FROG.
or condyles, instead of one, the ribs are either rudimentary or altogether wanting, and the sternum or breast-bone is imperfectly developed, or absent.
“To any person,” observes Professor Bell, “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 the 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 before alluded to, and to that of progressive development through the whole of the animal kingdom. That such phenomena are exhibited by the typical forms of this Class, will be sufficiently established by the slight sketch of their structure, habits, and development, which will presently be offered.”[1]
The Amphibia are arranged by the eminent zoologist just quoted in five Orders, distinguished by the general form, the presence and number of the limbs, and the character of the respiration. They are named Anoura, Urodela, Amphispneusta, Abranchia, and Apoda.
- ↑ “British Reptiles,” 72.