General features:
- They are commonly known as the creeping vertebrates.
- Reptiles were the first vertebrates fully adapted for life on land. Some live in water but they too return on land for breeding.
- Natural history:
- Reptiles originated from the salamander-like terrestrial amphibians before the Permian period.
- Mesozoic era is regarded as the age of reptiles.
- Most of the reptiles are carnivorous or insectivorous. Tortoises are herbivorous.
- Body temperature:
- They are poikilothermic (cold-blooded) animals.
- Most reptiles maintain a body temperature much higher than their surroundings, but they are ectothermic, getting most of their heat from the environment.
- The lie in the sun for some time before they warm up to be active.
- Body structure:
- The body varies in form and structure which may be short and broad or long and narrow, depressed or cylindrical.
- It is divisible into head, neck, trunk and tail.
- There are two pairs of pentadactyl limbs, each with 5 digits bearing horny claws.
- The limbs are directed outward and the animals creep on their belly.
- The limbs are absent in few lizards and all snakes.
- Skin:
- The skin is rough, and dry (without glands).
- An exoskeleton of horny epidermal scales is always present which is periodically sloughed off in pieces or as a whole and the process is called moulting.
- Skin contains a waterproof protein keratin, and checks the loss of water, enabling reptiles to live on dry land.
- Bony dermal plates may also occur beneath the scales.
*Keratinized skin in vertebrates is analogous of the chitinized cuticle of insects and the waxy cuticle of land plants.
- Endoskeleton:
- The endoskeleton is bony and skull is monocondylic, i.e it has a single occipital condyle.
- First two vertebrae, called atlas and axis are specialized to permit the head to move independently of the body.
- There are two sacral vertebrae.
- Digestive system:
- The mouth is large and usually armed with teeth in both the jaws.
- Tongue may or may not be protrusible.
- Distinct liver and pancreas are present and alimentary canal leads into the cloaca.
- Respiratory and circulatory systems:
- Respiration takes place by lungs.
- Ribs help in expansion and contraction of trunk, making respiration through lungs more efficient than in amphibians.
- The heart is incompletely four-chambered, having two auricles and a partly divided ventricle.
- The oxygenated blood from the lungs and the deoxygenated blood from the rest of the body don’t completely mix up in the ventricle.
- Sinus venosus is present, but truncus arteriosus is absent.
- Crocodiles have a completely four-chambered heart like the birds and mammals.
- Renal portal system is reduced. RBCs are oval, biconvex and nucleated.
- Sense organs:
- The olfactory sacs communicate with the anterior part of the buccopharyngeal cavity by internal nares.
- Ear has a single auditory ossicle. External ear may be present.
- There are 12 pairs of cranial nerves.
- Excretory system:
- Kidneys are metanephric and the waste material removed chiefly is uric acid in land forms (uricotelic) and as urea in aquatic forms (ureotelic).
- Reproduction:
- The gonoducts lead into the cloaca and fertilization is internal. Males generally have copulatory organs.
- Most forms are oviparous, some are ovoviviparous.
- Eggs are fewer, but large with abundant yolk and leathery or limy shell which are laid on dry land.
- Embryonic membranes, called allantoin and chorion, are formed during the development.
- Two membranous sacs are attached to the embryo: yolk sac that contains embryo’s food and allantois which stores the embryo’s nitrogenous waste until hatching.
- The chorion surrounds the embryo, amnion, yolk sac and allantois, and controls the overall permeability of the egg.
- The egg is permeable to gases but not to water. The egg which forms amnion is called amniotic egg.