Characteristic features of class Reptilia

Class Reptilia

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.
  1. 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.
  1. 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.
  1. 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.
  1. 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.

  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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).
  1. 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.

Characteristic features of class Reptilia