Reptiles

The Plesiosaurs carried this principle to its limit ; in them each limb is a rigid oar, flattened and widened distally, circular in section where attached to the body. It was feathered when brought forward, then turned so that its broad plane was vertical for the swimming stroke.

The crocodiles, the semi-marine lizard Amblyrhynchus, and the sea snakes are the living exponents of the second mode of swim ming. In them the powerful tail is laterally flattened, and is swung from side to side so that a series of waves passes along its length. The limbs are used for steering and for maintaining sta bility in the water. The only groups of reptiles which have formed a caudal fin are the Ichthyosaurs and the marine crocodiles of the family Metriorhynchidae. In each case the end of the vertebral column is suddenly turned down so that it passes into the lower lobe of a forked fin whose upper lobe is supported only by non ossified structures. These animals show a reduction of the limbs, the pelvic limb of Ichthyosaurs and the pectoral limb of the croco diles being reduced to a tiny paddle.

Limblessness.—Many lizards belonging to unrelated families, but chiefly of burrowing or sand living habits, exhibit a reduction of limbs associated with an elongation of the body. The process takes place gradually, all stages being known in one or other form between normally developed limbs and their complete absence.

In snakes the reduction is always complete in the case of the fore limbs, whilst the hind limbs may be represented by a claw like spur on each side of the vent. In some cases all three bones of the pelvis and the femur may be present. Normally, all trace of limbs, except for a rudimentary nerve plexus, is lost.

Skin.

The fact that the reptiles were originally distinguished from the amphibians by their more completely terrestrial habits, implies that in them the skin had become capable of withstand ing desiccation, and the serious wear to which it became exposed. The skin of amphibians is maintained in good condition by a coat of mucus, poured out from glands which lie all over it; that of reptiles is dry and covered by a watertight layer of horn, very well adapted for resisting abrasion. The horny layer, though continuous, is not of the same thickness throughout, but is divided into specially thickened areas, the scales which are connected by flexible regions. The scales may be flat, fitting together like a

mosaic or separated widely, or they may be prolonged backwards so that they overlap and are overlapped by others, like slates on a roof. The scales often have a definite arrangement, which is used in the classification of Squamata.

The skin, as in other Amniotes, consists of a compound squam ous epithelium which rests on a corium built of connective tissues.

The actual scale consists of the keratinized outer layers of the epidermis, its thickness is increased by additions to its inner sur face, and it grows in area either all round or at one end. The area of the scale is always raised by a special papilla of the corium, which may project so far that the scale overlaps that behind it. The scale is colourless, its transparency allowing the pigment in the cutis to show through.

The outer layer of the keratinized epithelium is worn away in crocodiles and Chelonia, but in the Rhynchocephalia and Squa mata it comes away either in flakes or, in some lizards and snakes, in one piece. Such cast skins exhibit perfectly the continuity of the horny skin, which in them even covers the eyes. This proc ess of shedding the skin is facilitated in some or all of these reptiles by a special mechanism which allows the head to be dis tended with blood. The papilla of the corium which fills the centre of each scale may, in crocodiles, some lizards and many fossil reptiles, be ossified as a bony scute.

The carapace and plastron of the Chelonia consist essentially of such scutes. Each ossifies in the corium, the bone finally occupying nearly the whole thickness of that layer, leaving only a thin sheet of connective tissue to support the peritoneum, and a similar sheet containing pigment cells below the epidermis in which the horny shields are developed. The originally dermal ossifications of the carapace extend so far down into the body of the animal that, in the end, they completely surround the middle parts of the ribs, which first calcify and are then ossified by ex tension of membrane bone from the scute.