DEFINITION:
Reptiles are one of the six basic groups of animals. Reptiles are cold-blooded vertebrates that
diverged from ancestral amphibians about 340 million years ago. There are two
characteristics that distinguished early reptiles from amphibians and enabled them
to colonize terrestrial habitats more extensively than their ancestors, scales and the
ability to lay hard-shelled eggs. Scales protect reptiles from abrasion and loss of
body moisture. Hard-shelled eggs provide a protective environment in which the
embryo can develop.
Reptile Characteristics
Reptiles are vertebrates (with a backbone), which can breathe air and are cold-
blooded (cannot regulate body temperature). They are considered as tetrapods
(with four legs or leg-like appendages), even though legs are absent in some of
these animals, like snakes. Their bodies are covered with scales or scutes and
they lay amniotic eggs (embryos develop inside amniotic membranes) on land.
They share the characteristics of both amphibians and mammals, but are
different and unique. For example, their skin is unique in the sense that it lacks
feathers, as in the case of birds; it does not have hair, like that of mammals and
it is not moist as in amphibians. One of the unique characteristics of reptiles is
their skin, which is tough and scaly. The following are some of the reptile
characteristics.
As mentioned earlier, reptiles have a dry skin with scales or scutes. The skin has
a few cutaneous glands and high levels of keratin, which prevents water loss
through the skin. The scales and scutes, are formed from the epidermis and are
also made of keratin, to protect the body.
These animals are considered tetrapods with two sets of paired limbs. Most of
these animals have five clawed toes on each limb. These muscular appendages
are angled downward to facilitate faster locomotion. In some reptiles, like
snakes and worm lizards, the legs are absent, but it is believed that these
animals evolve from some tetrapod ancestor.
All reptiles have spinal columns and a strong skeletal system with a rib cage.
They have a well-developed brain and a central nervous system. A pelvic region
with a minimum of two spinal bones is also seen in most of these animals.
Unlike amphibians, reptiles do not pass through an embryonic stage with gills.
These animals breath with well-developed lungs, right from birth. Most of them
have two lungs, except some snakes, which possess only a single lung. They
don't possess gills and even cutaneous respiration is very negligible.
All reptiles have three-chambered hearts, except crocodiles, which have four-
chambered hearts (2 atria, 2 ventricles), like mammals and birds. The three
chambers consist of two atria to receive blood and one partially divided
ventricle for pumping blood.
Reptiles have a digestive system, which is similar to other vertebrates, but the
gut, the ducts of urinary and sexual organs lead to one posterior chamber,
called the 'cloaca', which has a muscular opening at the base of the tail. In case
of those, who inhabit arid regions, water is further absorbed into the body from
the waste in the cloaca and only the remaining waste is excreted.
As far as reproduction is concerned, reptiles are unique, because they are the
first animals with amniotic eggs, that can be laid on land and not in water.
These amniotic eggs can survive without water, as they have protective shells
and membranes, that allow oxygen and other gases to enter. Most of the
reptiles lay eggs, but some of them give birth to young ones, by hatching the
eggs inside the body of the mother. Reptile characteristics include internal
fertilization, as sperm gets deposited into the reproductive tract of the female
directly.
The offspring of reptiles resemble the adults at the time of birth itself. There is
no metamorphosis, as in the case of amphibians.
Being cold-blooded, their body temperature vary with the surrounding
atmosphere. Like in mammals, the body temperature of reptiles is not
regulated by internal mechanisms. So they have to maintain the body
temperature by basking in the sun. This is the reason behind the presence of
many reptiles in arid regions, like deserts. However, they can be seen in
different habitats and if the temperature levels are not favorable, some of the
reptiles prefer hibernation too.
Physical characteristics of reptiles include their keen sense organs, which help
them to find food and escape from predators. Eyes are one of the most
important sense organ and in most reptiles, these organs are located at the
front of the head to facilitate binocular vision. While most of the different types
of lizards can move each eye independently, some of them have a protective
cover above the eyes. Most of them don't have external ears and the eardrum
is located near the eyes.
In short, these animals can be seen in diverse habitats, ranging from ponds,
seas, lakes, treetops, deserts and mountain ranges, but they are abundant in
deserts and arid regions too. The only limitation for them is that they are cold-
blooded and cannot generate body heat of their own. This is the reason behind
the absence of reptiles in the Antarctic region,
Examples:
crocodile
Aquatic and land reptile found in hot regions; it has an elongated
head, strong jaws, short legs and a powerful tail.
alligator
Short-legged aquatic reptile found in North America and China; its
head is shorter and wider than that of the crocodile.
caiman
Medium-sized aquatic reptile found in Central and South America;
it is less aggressive than the crocodile and the alligator.
