A reptile is an air-breathing, cold-blooded, egg-laying vertebrate with an outer covering of scales or plates and a bony skeleton; it is a member
of the class Reptilia. Lizards, crocodiles, tortoises,
and snakes are all reptiles. For example, Lacertilla, also called Sauria, is a suborder (order Squamata)
of the reptiles that includes the lizards.
Lizards usually have well-developed limbs and the nearly 3,800 known species
are found mainly in tropical and subtropical regions.
|The Nile crocodile, one of the largest
Body form and skeleton
Most reptiles have elongated bodies, like those of lizards, crocodiles,
and snakes, but the bodies of tortoises and turtles are short and broad.
The limbs, when present, are usually short. There are numerous vertebrae:
about 60 in crocodiles and as many as 400 in snakes. The head is held off
the ground and the neck is more developed than in amphibians, though the
plan of the skeleton is similar to that of amphibians. Ribs are well developed
between the shoulder and the hip region. Reptiles have more powerful jaws
than do amphibians, the lower being made up of several bones. There are
usually openings in the temporal region of the skull.
|Reptilian body forms
Limbs and digits
The limbs of reptiles (absent in snakes) are jointed and typically five-fingered.
Five is the primitive number of digits on the limbs of vertebrate animals,
and most reptiles have this number, though crocodiles have only four on
their hind feet. The digits are nearly always armed with claws. The limbs
of the marine turtle take the form of paddles.
|The feet of various types of reptiles
|A lizard's legs push it along but
do not support its body
The limbs of most reptiles project on each side of the body, so that the
animal does not stand up on its legs but crawls with its belly touching
the ground; some lizards, however, run actively on their hind legs. Snakes
have no limbs and progress by crawling, and there are also certain lizards,
like the slow-worm, which are limbless.
|How a reptile's legs are joined to its body
Reptiles differ from amphibians in having skin which is usually covered
with horny scales or plates. They may also have bony shields embedded in
their skin, as in the crocodiles, and, in more exaggerated form, in tortoises
and turtles. Another distinction is that the skin of reptiles, at any rate
over most of their bodies, is not glandular, so that it is not moist or
slippery like that of a frog. A snake's skin is dry and clean, not slimy
as some people believe.
|The scaly head of a snake
Tortoises and turtles have sharp, horny edges to their jaws, but no teeth.
The teeth of other reptiles are usually sharp and pointed. They are never
differentiated into incisors, canines, and molars like the teeth of mammals,
but some snakes have hollow fangs, used for injecting poison, as well as
ordinary teeth. Reptiles' teeth are used for grasping and tearing, and they
swallow food whole or in lumps; it is then dissolved by powerful digestive
Nose and ears
The nostrils of reptiles are situated on each side of their muzzles, and
their sense of smell is well developed. Snakes
can also be said to "smell with their tongues," as they dart them in and
out, collecting scent molecules from the air which are conveyed to a special
sense organ in the mouth. The external ear of most reptiles is a simple
opening in the head, but this is not present in snakes, which are deaf to
All reptiles breathe air by means of lungs. Generally they have two lungs,
like other land vertebrates. In the more primitive snakes, however, such
as pythons, the left lung is reduced in size, while in most other species
of snakes it is absent, and only the right lung is developed. This is probably
an adaptation to their slender form. The lungs of reptiles are less highly
developed, and less efficient than those of birds and mammals.
Circulation of the blood
Most reptiles have hearts divided into three chambers – two atria
and one ventricle – and have two systemic arches. This is different
from mammals and birds, in which the ventricle is divided so that the heart
consists of four chambers; and which have only one systemic arch (the right
in birds, the left in mammals).
In the diagram on the right, blood vessels carrying blood charged with oxygen
(arterial blood) are colored red, those carrying blood without oxygen (venous
blood) are blue. The top diagram shows venous blood coming through two veins
into the right atrium, while the left atrium is filled with arterial blood
from the lungs. In the ventricle the two kinds of blood become partly mixed.
Three arteries leave the ventricle: the pulmonary artery carries venous
blood to the lungs; the right systemic arch pure arterial blood; and the
left systemic arch mixed blood. These arches join and supply blood to the
system; this is not therefore pure arterial blood as it is in the circulation
of birds and mammals.
In the crocodiles (lower diagram) the ventricle is divided, but as the left
systemic arch still functions, the animal does not gain full advantage from
this. But the left arch is narrow, and the two arches are joined by an opening,
the foramen of Panizza; this limits mixing of the blood and shows an advance
on the normal reptilian pattern.
The body temperature of reptiles is not kept automatically at a constant
(and rather high) level like that of birds and mammals. The blood of a lizard
basking in the hot sun gets quite warm, but at night, or on a cold day,
it is much cooler. When reptiles are cold they become sluggish, and in temperate
climates they cannot remain active in winter, but have to hibernate.
For this reason far more kinds of reptiles are found in the tropics than
in temperate and cold regions.
The brains of reptiles are of the same pattern as those of fishes and amphibians, but their fore-brains are considerably larger. It is this
part of the brain that in mammals, including humans, becomes convoluted
and forms the cerebral hemispheres.
|Brain of a crocodile from the left
|A baby crocodile hatching from the
Breeding and birth
Reptiles breed on land and their young grow inside eggs. (In contrast, amphibians have to return to water to breed.) They are either oviparous or ovoviviparous.
Oviparous means "egg-laying," and most reptiles reproduce in this
way. The covering of their eggs is usually a parchment-like skin, but crocodiles
and the lizards called geckos lay eggs within a hard shell.
Viviparous is the word applied to animals that bear their young alive, as
mammals do. Some snakes bear living young, but these have developed in the
mother's body enclosed in a membrane and nourished by a yolk, just as embryos
develop in an egg. Water and oxygen are supplied to the developing young,
but their blood circulation is not continuous with the mother's as in mammals.
This type of reproduction is called ovoviviparous.
Evolution of reptiles
|A reptile evolutionary tree indicates how few species survived from the reptiles' heyday. Crocodiles and alligators are the closest living relations to dinosaurs and also have the most advanced brains and hearts. Turtles and tortoises have changed very little over millions of years; like many areas of reptilian evolution, their origins are obscure. Therapsids, though all species are now extinct, are a vital branch of evolution, for it is from them that the synapsids evolved, which in turn led to the mammals. All, however, evolved from the earliest group of reptiles, the captorhinomorphs, otherwise known as the "stem reptiles."