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.
(1857). The globe that we inhabit is usually said to be made up of land and water; and perhaps, for the purposes of the geographer, such a division of the surface of our planet is all that is requisite. A slight investigation of this subject, however, is sufficient to convince the naturalist that a very considerable proportion of the world around us can scarcely be strictly referred to either one or the other of the geographical sections alluded to, - that there are extensive marshes, for instance, equally ill-adapted to be the habitation of aquatic animals and of creatures organized for a purely terrestrial existence; that some localities may be alternately deluged with water and parched with drought; that the margins of our lakes, the banks of our rivers, and the shallow ponds and streamlets of warm climates could only be adequately populated by beings of an amphibious character, alike capable of living in an aquatic or in an aerial medium, and combining in their structure the conditions necessary for enabling them to reside in either element.
(1858). Aquatic animals, strictly so called, breathe by means of gills. For a vertebrate animal to respire air, it must be provided with lungs; but if a creature is destined to live both in air and water, it must obviously have both gills and lungs coexistent, either of which may be employed in conformity with the changing necessities and altered circumstances. We therefore cannot be surprised to find that in the lowest Reptiles this is literally the arrangement adopted - that they respire, like fishes, by means of branchiae while in the water, whereas on emerging into the air they have lungs ready for use.
(1859). The Amphibia (Batrachia, Cuv.) are to the anatomist amongst the most interesting animals in the whole range of zoology, as we trust will be made sufficiently evident when we come to investigate their internal economy; but it is to their outward forms and habits that we must first introduce the reader, leaving the details of their organization to be discussed in the sequel.
(1860). From whatever form or race of animals the zoologist advances towards the next succeeding it in the great scale of Nature, he will find himself insensibly led on by such gentle gradations that the transition from any one class to another is almost imperceptible. Nihil per saltum is one of the most obvious laws in creation; and of this, perhaps, we could not select a more striking illustration than is afforded by the Lepi-dosiren (fig. 328).
(1861). Two distinct species of this most remarkable animal have been met with: one, the Lepidosiren paradoxa, discovered by Dr. Nat-terer in the river Amazon; the other, Lepidosiren annectam, was found by T. C. B. Weir, Esq., and is a native of the African continent, inhabiting the river Gambia. An individual of the species last mentioned has been minutely anatomized by Professor Owen*, and both in its outward form and internal organization is so precisely intermediate between a Reptile and a Fish, that, while Dr. Natterer regards it as an Amphibian, Professor Owen considers that, notwithstanding that it possesses lungs, the ichthyic characters predominate, and it ought rather to be ranked among the Fishes.
(1862). The body of the Lepidosiren annectans (fig. 328) is about a foot long, and covered with scales, resembling those of the cycloid fishes; the tail gradually tapers to a point, but is fringed above and below with a membranous fin supported by numerous soft, elastic, transparent rays, articulated to the superior and inferior spines of the caudal vertebrae; the gills are covered by opercula - not being exposed, as in the proper Amphibia; and, moreover, it has four rudimentary fins or legs, as the reader may choose to call them. These rudimental extremities are round, filiform, and gradually attenuated to an undivided point, being supported internally by a single-jointed soft or cartilaginous ray. The nostrils of the Lepidosiren, however, are merely two blind sacs, as in fishes, and do not communicate with the mouth or fauces - a character which Professor Owen regards as the only decided evidence that the animal ought in preference to be ranked among the class Pisces.
(1863). The Siren lacertina, a creature which inhabits the marshes of Carolina, is another amphibious animal, scarcely further removed from the Fishes than the last. This Siren attains the length of two or three feet; it has a body very nearly resembling that of an eel; but instead of pectoral fins it has two rudimentary feet, each provided with four fingers, - its hind feet, the representatives of the ventral fins, being entirely wanting; it is moreover furnished with gills placed on each side of the neck, while internally it possesses two capacious membranous lungs adapted to aerial respiration.
(1864). In the Proteus anguinus, an animal only met with in the subterranean waters of Carniola, the body, of which a figure is given in a subsequent page (fig. 340), is equally anguilliform; but the legs are now four in number, although still very imperfectly developed. Its gills are fringes of blood-vessels placed externally upon the sides of the neck, and its thin and delicate lungs (t, z) extend nearly the whole length of the abdomen.
* Transactions of the Linnean Society for 1840.
(1865). The Amphibia above-mentioned, as well as the Menobranchus and the Aocolotl, both animals of very similar construction, preserve their branchiae through the whole period of their lives, and are for this reason denominated Amphibia perennibranchiata. But there are other genera which, although in the early part of their existence they are equally provided with both gills and lungs, ultimately become sufficiently perfect in their organization to enable them to enjoy a more or less complete terrestrial existence, and consequently their branchiae become obliterated as the lungs grow more efficient, until at length no vestiges of the former remain perceptible. These are called A. caduci-branchiata.