[4] By some naturalists the _Amphibia_ are considered as a distinct class, by other as a sub-class either of _Reptilia_ or of _Pisces_. Of the reptiles proper (at present existing), the arrangement into the orders _Testudinata_ (or Tortoises), _Sauria_ (or Lizards), and _Ophidia_ (or Snakes), is the one most generally adopted; but De Blainville elevates the _Loricata_ (or Crocodiles) to the rank of an order, and others have adopted a division of corresponding rank, _Saurophidia_, for the _Anguis_ series above referred to; but the latter are merely limbless Lizards (or with abortive limbs) akin to the Scinques.--ED. CHAPTER I. AMPHIBIA, OR BATRACHIANS. Those geographers who divide the world into land and sea overlook in their nomenclature the extensive geographical areas which belong permanently to neither section--namely, the vast marshy regions on the margins of lakes, rivers, and ponds, which are alternately deluged with the overflow of the adjacent waters, and parched and withering under the exhalations of a summer heat; regions which could only be inhabited by beings capable of living on land or in water; beings having both gills through which they may breathe in water, and lungs through which they may respire the common air. The first order of reptiles possesses this character, and hence its name of Amphibia, from +amphibios+, having a double life. The transition from fishes to reptiles is described by Professor Owen, with that wonderful power of condensation which he possesses, in the following terms:--"All vertebrates during more or less of their developmental life-period float in a liquid of similar specific gravity to themselves. A large proportion, constituting the lowest organised and first developed forms of this province, exist and breathe in water, and are called fishes. Of these a few retain the primitive vermiform condition, and develop no limbs; in the rest they are 'fins' of simple form, moving by one joint upon the body, rarely adapted for any other function than the impulse or guidance of the body through the water. The shape of the body is usually adapted for moving with least resistance through the liquid medium. The surface of the body is either smooth and lubricous or it is smoothly covered with overlapping scales; it is rarely defended by bony plates, or roughened by tubercles. Still more rarely it is armed with spines." Passing over the general economy of fishes we come to the heart. "The heart," he tells us, "consists of one auricle receiving the venous blood, and one ventricle propelling it to the gills or organs submitting that blood in a state of minute subdivisions to the action of aërated water. From the gills the aërated blood is carried over the entire body by vessels, the circulation being aided by the contraction of the surrounding muscles."