THE remarkable beings which are classed together under the general title of REPTILES, or creeping animals, are spread over those portions of the globe where the climate is tolerably warm. and are found in the greatest profusion under the hotter latitudes. Impatient of cold, though capable of sustaining a temperature of such freezing chilliness

that any of the higher animals would perish under its severity, and for the most part being lovers of wet and swampy situations, the Reptiles swarm within the regions near the equator, and in the rivers or vast morasses of the tropical countries the very soil appears to teem with their strange and varied forms. Indeed, the number of Reptiles to be found in any country is roughly indicated by the parallels of latitude, the lands near the equator being the most prolific in these creatures, and containing fewer as they recede towards the poles.

Some Reptiles inhabit the dry and burning deserts; but the generality of these creatures are semi-aquatic in their habits, are fitted by their structure for progression on land or in water, and are able to pass a considerable time below the surface without requiring to breathe.

This capacity is mostly the result of the manner in which the circulation and aeration of their blood is effected.

As has been shown in the two volumes on Mammalia and Birds, the heart in these animals is divided into a double set of compartments, technically termed auricles and ventricles, each set having no direct communication with the other. In the Reptiles, however, this structure is considerably modified, the arterial and venous blood finding a communication either within or just outside the two ventricles, so that the blood is never so perfectly aerated as in the higher animals. The blood is consequently much colder than in the creatures where the oxygen obtains a freer access to its particles.

In consequence of this organization the whole character of the Reptiles is widely different from that of the higher animals. Dull sluggishness seems to be the general character of a Reptile, for though there are some species which whisk about with lightning speed, and others, especially the larger lizards, can be lashed into a state of terrific frenzy by love, rage, or hunger, their ordinary movements are inert, their gestures express no feeling, and their eyes, though bright, are stony, cold, and passionless. Their mode of feeding accords with the general habits of their bodies, and the process of digestion is peculiarly slow.

Most of the Reptiles possess four legs, but are not supported wholly upon them, their bellies reaching the ground and being dragged along by the limbs. One or two species can support themselves in the air while passing from one tree to another, much after the fashion of the flying squirrels; and in former days. when Reptiles were apparently the highest race on the surface of the earth, certain species were furnished with wing-like developments of limb and skin, and could apparently flap their way along like the bats of the present time.

Excepting some of the tortoise tribe, the Reptiles are carnivorous beings, and many of them, such as the crocodiles and alligators, are among the most terrible of rapacious creatures. In this class of animals we find the first examples of structures which transmute Nature's harmless gifts into poison, a capacity which is very common in the later orders, such as the spiders and insects, and is developed to a terrible extent in some of the very lowest beings that possess animal life, rendering them most formidable even

to man.

The skeleton of a true Reptile, from which class the Batrachians, i.e. the frogs, salamanders, and their kin are excluded, for reasons which will presently be given, is composed of well-ossified bones, and is peculiarly valuable to the physiologist. It is well known to all who have studied the rudiments of anatomy, that each bone is formed from several centres, so to speak, consisting of mere cartilaginous substance at its earliest formation, and becoming gradually ossified from several spots.

In the young of the higher animals these centres are only seen during their very earliest stages, and are by degrees so fused together that all trace of them is obliterated. But in the Reptiles it is found that many of the bones either remain in their separate parts, or leave so distinct a mark at the place where they unite, that their shape and dimensions are clearly shown. In the head of the adult crocodile, for example, the frontal bone is composed of five distinct pieces, the temporal of at least five pieces, and each side of the lower jaw-bone is composed of either five or six portions united by

sutures.

SKELETON OF CROCODILE.

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With the exception of the tortoises, the Reptiles mostly possess a goodly array of teeth, set in the jaw or palate, and as a general fact, being sharp and more or less curved backward. Their bodies are covered with various modifications of the structure termed the dermal, i.e. skin skeleton, and are furnished with scales and plates of different forms. In some cases the scales lie overlapping each other like those of the fish, in others they are modified into knobby plates, and in some, of which the tortoises afford well-known examples, they form large flat plates on the back and breast, and scales upon the feet and legs.

The young of Reptiles are produced from eggs, mostly being hatched after they have been laid, but in some cases the young escape from the eggs before they make their appearance in the world. As a general fact, however, the eggs of Reptiles are placed in some convenient spot, where they are hatched by the heat of the sun. Some species are

SKELETON OF CROCODILE

very jealous about their eggs, keeping a strict watch over them, and several of the larger serpents have a curious fashion of laying the eggs in a heap, and then coiling themselves around them in a great hollow cone. The size of the eggs is extremely variable, for, although as a general fact those of the smaller Reptiles are large in proportion to the dimensions of the parent, those of the crocodiles and alligators are wonderfully small, not larger than those of our domestic geese, and in many cases much smaller. They are usually of a dull white colour, and in some instances are without a brittle shell, their covering being of a tough leathery consistence.

In form, and often in colour, the Reptiles exhibit an inexhaustible variety, and even each order displays a diversity of outward aspect unexampled in the two previous classes of Mammals and Birds. Strange, grotesque, and oftentimes most repulsive in appearance, though sometimes adorned with the brightest tints, the Reptiles excite an instinctive repugnance in the human breast; and whether it be a lizard, a snake, or a tortoise, the sudden and unsuspected contact of one of these beings will cause even the most habituated to recoil from its cold touch. This antipathy may, perhaps, have some connexion with the instinctive association of cold with death; but whatever may be the cause, the feeling is deep and universal.

TORTOISES.

THE very curious reptiles which are known by the general name of Tortoises, are remarkable for affording the first example of a skeleton brought to the exterior of the body, a formation which is frequent enough in the lower orders, the crustaceans and insects

SKELETON OF TORTOISE

being familiar examples thereof. In these reptiles the bones of the chest are developed into a curious kind of box, more or less perfect, which contains within itself all the muscles and the viscera, and in most cases can receive into its cavity the head, neck, and limbs; in one genus so effectually, that when the animal has withdrawn its limbs and head, it is contained in a tightly closed case without any apparent opening.

The shell of the Tortoise is divided into two portions, the upper being termed the carapace, and the lower the plastron.

The carapace is formed by a remarkable development of the vertebræ and ribs, which throw out flat processes, and are joined together by sutures like the bones of the skull. The back is therefore incapable of movement, and from the arched shape of the bones is wonderfully strong when resting on the ground. In the Tortoises these bones are united throughout their entire length, but in the Turtles the ends of the ribs retain their original width.

The plastron is similarly formed of the breastbone, which is thought in these creatures to be developed to the greatest extent of which it is capable. It is composed of nine pieces, each being formed from one of the bony centres already mentioned. These bones. are arranged in four pairs, and one in the centre of the front.

As all the limbs have to be worked from the interior of the chest, amid the vital organs and muscles for moving them, they undergo considerable modification. The shoulder-blade, for example, is a curious three-branched bone, quite unique among

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