Reptiles A.) External Structure ­Reptiles are tetrapod vertebrates, creatures that either have four limbs or, like snakes, being descended from four­limbed ancestors.
­Reptilian skin is covered in a horny epidermis, making it watertight and enabling reptiles to live on dry land, in contrast to amphibians.
­ reptiles skin is rather thin and lacks the thick dermal layer that produces leather in mammals
­Exposed parts of reptiles are protected by scales or scutes, sometimes with a bony base, forming armor
B.)Locomotion.)
­Crocodiles and alligators, turtles, most lizards, and tuataras can walk on their four legs.
­Although not all reptiles have legs, many of them do.
­Usually they walk with their legs held out from the body, rather like a human would hold up his or her body when doing a push­up.
­Many of the smaller lizards, in particular, are very speedy, zipping across the ground at speeds that make their capture difficult.
C.) Nutriton and Digestive System
­Most reptiles are insectivorous or carnivorous and have rather simple and comparatively short digestive tracts, meat being fairly simple to break down and digest
­Digestion is slower than in mammals, reflecting their lower resting metabolism and their inability to divide and masticate their food
­Some reptiles have sticky tongues for capturing prey
­the glottis is far forward so that the snake can breathe while slowing its prey
D.) Gas Exchange and circulatory system
­Reptiles exchange respiratory gases across internal respiratory surfaces to avoid losing large amounts of water
­ lung chambers provide a large surface area for gas exchange
­reptiles possess two atria that are completely separated in the adult and has veins from the body and lungs emptying into them
­they stop breathing during diving.
E.) Temperature Regulation
­ the ventricle of most reptiles is completely divided
­the ventral aorta and conus arteriosclerosis divide during development and become three major arteries that leave the heart
­blood low in oxygen enters the ventricle from the lungs via pulmonary veins
F.) Nervous and Sensory Functions
­The cerebral hemispheres are somewhat larger than those of amphibians
­The increased size is associated with an improved sense of smell
­the optic lobes and cerebellum are also enlarged which reflects increased reliance on vision and more coordinated muscle functions
­the ears of snakes detect substrate vibrations
G.)Excretion and Osmoregulation

­most reptiles excrete Uric acid
­the kidneys are similar to fishes and amphibians
­the urinary bladder or cloacal walls reabsorb water and Uric acid can be stored