The second great division of the Vertebrate sub-kingdom, according to Huxley, is that of the Sauropsida, comprising the true Reptiles and the Birds. It is, no doubt, at first sight an almost incredible thing that there should be any near bond of relationship between the Birds and the Reptiles, no two classes of animals being more unlike one another in habits and external appearance. It is, nevertheless, the fact that the Birds are more nearly related to the Reptiles than to any other class of the Vertebrata, and it will shortly be seen that many affinities and even transitional forms are known to exist between these great sections. The Reptiles and Birds, then, may be naturally included in a single primary section of Vertebrates, which may be called Sauropsida after Huxley, and which is defined by the possession of the following characters: - At no period of existence are branchiae, or water-breathing respiratory organs, developed upon the visceral arches ; the embryo is furnished with a well-developed amnion and allantois; the red corpuscles of the blood are nucleated (fig. 245, b, c); the skull articulates with the vertebral column by means of a single articulating surface or condyle; and each half or "ramus" of the lower jaw is composed of several pieces, and articulates with the skull, not directly, but by the intervention of a peculiar bone, called the "quadrate bone," or " os quadratum " (fig. 288).

These being the common characters of Reptiles and Birds, by which they are collectively distinguished from other Vertebrates, it remains to inquire what are the characters by which they are distinguished from one another. The following, then, are the characters which separate the Reptiles from the Birds : - The blood in Reptiles is cold - that is to say, slightly warmer than the external medium - owing mainly to the fact that the pulmonary and systemic circulations are always directly connected together, either within the heart or in its immediate neighbourhood, so that the body is supplied with a mixture of venous and arterial blood, in place of pure arterial blood alone. The terminations of the bronchi at the surface of the lung are closed, and do not communicate with air-sacs, placed in different parts of the body. When the epidermis develops horny structures, these are in the form of horny plates or scales, and never in the form of feathers. The fore-limbs are formed for various purposes, including in some cases even flight, but they are never constructed upon the type of the "wing" of Birds. Lastly, with one or two doubtful exceptions, whilst the ankle-joint is placed between the distal and proximal portions of the tarsus, the tarsal and metatarsal bones of the hind-limb are never anchylosed into a single bone.

These are the leading characters by which Reptiles are distinguished from Birds, but we must not forget the other distinctive peculiarities in which Reptiles agree with Birds, and differ from other Vertebrates - namely, the presence of an amnion and allantois in the embryo, the absence of branchiae at all times of life, the possession of only one occipital condyle, and the articulation of the complex lower jaw with the skull by means of a quadrate bone.

It is now necessary to consider these characteristics of the Reptilia a little more minutely. The class includes the Tortoises and Turtles, the Snakes, the Lizards, the Crocodiles, and a number of extinct forms; and, with the exception of the Tortoises and Turtles, they are mostly of an elongated cylindrical shape, provided posteriorly with a long tail. The limbs may be altogether absent, as in the Snakes, or quite rudimentary, as in some of the Lizards; but, as a general rule, both pairs of limbs are present, sometimes in the form of ambulatory legs, sometimes as swimming-paddles, and in some extinct forms modified to subserve an aerial life. The endoskeleton is always well ossified, and is never cartilaginous or semi-cartilaginous, as in many fishes and some Amphibians. The skull articulates with the atlas by a single condyle. The lower jaw is complex, each half or ramus being composed of from four to six pieces, united to one another by sutures (fig. 288). In the Tortoises, however, these are anchylosed into a single piece, and the two rami are also anchylosed. In most reptiles, however, the two rami of the lower jaw are only loosely united - in the Snakes by ligaments and muscles only, in the Lizards by fibro-cartilage, and in the Crocodilia by a regular suture. In all, the lower jaw articulates with the skull by a quadrate bone (fig. 288, a); and as this often projects backwards, the opening of the mouth is often very extensive, and may even extend beyond the base of the skull. Teeth are usually present, but are not sunk in separate sockets or alveoli, except in the Crocodiles, and in some extinct forms. In the Tortoises and Turtles alone of living types there are no teeth, and the jaws are simply sheathed in horn, constituting a kind of beak like that of a bird.

As regards the exoskeleton, most Reptiles have horny epidermic scales, and they are divided into two great sections - called respectively Squamata and Loricata - according as the integumentary skeleton consists simply of these scales, or there are osseous plates developed in the derma as well. In the Tortoises, the epidermic plates unite with the bony exoskeleton and with the true endoskeleton to form the case or box in which the body of these animals is enclosed.

Fig. 288.   Skull of a Serpent (Python), b Articular portion of the lower jaw ; a Quadrate bone ; c Squamosal portion of the temporal bone.

Fig. 288. - Skull of a Serpent (Python), b Articular portion of the lower jaw ; a Quadrate bone ; c Squamosal portion of the temporal bone.

The digestive system of the Reptilia possesses few characters of any special importance, except that the rectum opens, as in Amphibia, into a common cavity or "cloaca," which not only receives the faeces, but also serves for the discharge of the products of the urinary and generative organs.

The heart in the Reptiles consists of two completely separate auricles, and a ventricular cavity, which is divided into two by an incomplete partition. In the Crocodilia alone is the septum between the ventricles a perfect one; and even in these, as in all other Reptiles, the heart consists functionally of no more than three chambers. The ordinary course of the circulation, where the ventricular septum is imperfect, is as follows: - The impure venous blood returned from the body is, of course, poured by the venae cavae into the right auricle (fig. 289, a), and thence into the ventricle. The pure arterialised and aerated blood that has passed through the lungs, is, equally of course, poured into the left auricle (a'), and thence propelled into the ventricle (v). As the ventricular cavity is single, and not divided by a complete partition, it follows of necessity that there is a mixture in the ventricle, resulting in the production of a mixed fluid, consisting partly of venous and partly of arterial blood. This mixed fluid, then, occupies the common ventricular cavity, and by this it is driven both to the lungs (through the pulmonary artery), and to the body (through the systemic aorta). Consequently, in Reptiles, both the lungs and the various tissues and organs of the body are supplied with a mixture of arterial and venous blood, and not with unmixed blood - the lungs with purely venous, and the body with purely arterial blood - as is the case with the higher Vertebrata. In the Crocodilia, as before said, the partition between the ventricles is a complete one, and consequently this mixture of the arterial and venous blood cannot take place within the heart itself. In these Reptiles, however, a direct communication exists between the pulmonary artery and aorta (the right and left aortae) by the so-called "foramen Panizzae," close to the point where these vessels spring respectively from the right and left ventricle. In these Reptiles, therefore, the same mixture of arterial blood with venous takes place as in the lower Reptilia, though probably not to so complete an extent. It is this peculiarity of the circulation in all Reptiles which conditions their low temperature, slow respiration, and generally sluggish vital actions.

The lungs in all Reptiles, except the Crocodiles, are less completely cellular than in the Birds and Mammals, and they often attain a very great size. In no Reptile is the cavity of the thorax shut off from that of the abdomen by a complete muscular partition or "diaphragm;" though traces of this structure are found in the Crocodiles. The lungs, therefore, often extend along the whole length of the thoracico-abdominal cavity. In no case are the lungs connected with air-receptacles situated in different parts of the body; and not uncommonly there is only a single active lung, the other being rudimentary or completely atrophied (Ophidid).

Fig. 289.   Diagram of the circulation in Reptiles. (The venous system is left light, the arterial system is black, and the vessels containing mixed blood are cross shaded.) a Right auricle, receiving venous blood from the body; a' Left auricle, receiving arterial blood from the lungs; v Arterio venous ventricle, containing mixed blood, which is driven by (p) the pulmonary artery to the lungs, and by (o) the aorta to the body.

Fig. 289. - Diagram of the circulation in Reptiles. (The venous system is left light, the arterial system is black, and the vessels containing mixed blood are cross-shaded.) a Right auricle, receiving venous blood from the body; a' Left auricle, receiving arterial blood from the lungs; v Arterio-venous ventricle, containing mixed blood, which is driven by (p) the pulmonary artery to the lungs, and by (o) the aorta to the body.

Lastly, all Reptiles are essentially oviparous, but in some cases the eggs are retained within the body till the young are ready to be excluded, and the animals are then ovo-viviparous. The egg-shell is usually parchment-like, but sometimes contains more or less calcareous matter.