REPTILES (Reptilia) is the name given to a class of verte brates which hold a position in the animal kingdom intermediate between the amphibians and the birds, and the mammals. The group arose, perhaps in Lower Carboniferous times, from the Labyrinthodont Amphibia, and was already varied at the end of the Carboniferous. During Permian times the class branched out into many orders, one of which included the ancestors of the Mam malia, whilst from another the birds, crocodiles, Sphenodon, and perhaps the lizards and snakes arose. The tortoises are the de scendants of another early group. In a recent survey by Nopcsa, it is pointed out that, of the 125 families into which he divides the reptiles, only 18 are represented by living forms, whilst of the 19 orders only four are extant. The modern forms fall into the orders Crocodilia, including the crocodiles and alligators; the Squamata, the lizards and snakes; the Rhynchocephalia, repre sented only by the Tuatera lizard of New Zealand, and the Che lonia, the tortoises and turtles. These living forms are char acterized as follows: (I.) The animal breathes air by lungs.
(2.) The body temperature is variable.
(3.) The skin is covered with horny scales formed by the epi dermis.
(4.) Fertilization is internal, and an egg, consisting of a yolk surrounded by albumen and contained in a shell, is usually laid and hatched by the heat of the sun or of decaying vegetation. In some cases reptiles are viviparous.
(5.) In the brain the cerebral hemispheres are comparatively small. Their roof tends to become thinned and may be almost membranous. There is a well-developed hypopallium which be comes assimilated to the corpus striatum, losing the original strati fication of the neurones. The mid-brain is relatively large and its roof forms a pair of large optic lobes.
(6.) The olfactory organ has its surface increased by a simple turbinal or concha, and there is a well-developed Jacobson's organ.
The posterior nares may be immediately below the external nos trils or may be carried back to the hinder end of the head.
The eyes are usually present, but may be hidden in burrowing forms. The retina normally contains both rods and cones, but may consist exclusively of either type. There is a pecten in the form of a folded sheet projecting into the vitreous humour.
The internal ear shows a more marked separation of sacculus and utriculus than obtains in Amphibia, a lagena always occurs and is associated with a perilymphatic duct, in some cases so as to form a rudimentary cochlea. The tympanic cavity lies high up and the tympanic membrane is either superficial or lies at the end of a short external auditory meatus. The membrane is con nected to that closing the fenestra ovalis by a straight rod, whose inner end, the columella, is bony, whilst the outer half, the extra columella, is often four-rayed, a short dorsal process being con nected to the end of the paroccipital process and a ventral process often continued into the hyoid.
(7.) There is a well-developed tongue capable of free move ment. The mid-gut has the usual structure; there is a cloaca and an urinary bladder of allantoic origin. The lungs are more elabo rate than those of Amphibia and less than those of mammals.
(8.) The heart is three- or four-chambered, there being two auricles and a ventricle more or less completely divided into two. There is no bulbus ; three arteries arise from the ventricle; of these one is the right systemic, another the pulmonary, whilst the third is the left systemic and both carotids. The posterior cardinals have both almost disappeared as such, the post-caval vein returning most of the blood from the posterior part of the animal to the heart. There is a coronary circulation.
(9.) The functional kidney in the adult is a metanephros dis charging by a ureter into the cloaca. The ovary is often single and the egg always large. The oviduct is provided with glands which secrete albumen and a shell. A copulatory organ is usually present in the male, but is variable in structure.