Reptiles are tetrapodvertebrates, creatures that either have four limbs or, like snakes, are descended from four-limbed ancestors. Unlike amphibians, reptiles do not have an aquatic larval stage. Most reptiles are oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades – the fetus develops within the mother, using a (non-mammalian) placenta rather than contained in an eggshell. As amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, which can grow up to 17 mm (0.7 in) to the saltwater crocodile, Crocodylus porosus, which can reach 6 m (19.7 ft) in length and weigh over 1,000 kg (2,200 lb).
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Kidneys are metanephric and the waste material removed chiefly is uric acid in land forms (uricotelic) and as urea in aquatic forms (ureotelic).
- 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.