Reptiles
 
Reptilian Characteristics:
 
  • The development of the amniote egg. Reptiles are considered the first true land animals. It was this egg that allowed the reptiles to conquer the land and begin a reign that lasted hundreds of millions of years. The amniote egg is a water proof structure composed of a shell, embryonic membranes, and the living embryo. Starting after the shell, the first extraembryonic membrane is the chorion. The chorion aids in the exchange of gases between the environment and the embryo. The allantois is a sac-like structure that grows out of the digestive tract of the embryo. It functions in gas exchange and collects metabolic wastes from the embryo. The third membrane is a fluid filled sac called the amnion. It surrounds the embryo and the fluid it secretes cushions the embryo. The fourth and final membrane is the yolk sac. The sac contains the food for the embryo.
  • A thick dry skin covered with scales made of a protein called Keratin. Reptiles molt as they grow.
  • Reproduction is sexual with internal fertilization. Most are oviparous, while some snakes and lizards are either ovoviviparous or viviparous.
  • Respiration occurs through well developed lungs, and in some turtles a moist cloacal surface.
  • Reptiles are ectotherms. Being ectothermic enables an organism to survive on much less food than an endothermic organism.
  • Most reptiles contain a 3 chambered heart with a partially divided ventricle. Crocodiles contain a 4 chambered heart.
 
Reptilian Orders
  • Order Chelonia. Turtles are members of this order. They developed from animals called anapsids during the Mesozoic era. They show very little change from their ancestral form. They contain 2 shells. The top shell is called the carapace, while the lower shell is called the plastron. Both shells are colored to help the turtle blend in with the 2 environments in which it lives. All turtles lay their eggs on land. Turtles do not contain teeth, but use a horny beak to grab and tear food. Turtles contain three eyelids.
  • Order Squamata. Lizards and snakes belong to this reptilian order. Lizards are the most abundant of this order. They evolved from the diapsid group of primitive organisms. They range in size from the large Komodo Dragon to the small tropical geckos. Snakes are probably the most interesting species of this group. They probably developed from small burrowing lizards. They are void of legs, eyelids, and ears. They are carnivores swallowing their prey whole. Their glottis and gullet are at separate ends of the mouth cavity. This allows the snake to breathe when eating. They contain 2 rows of teeth on the upper jaw and one row on the lower. The teeth are curved and pointed back toward the gullet. This keeps the food moving in one direction toward the stomach. The jaws are hinged in such a way as to separate allowing the mouth to open wider than usual when swallowing large prey. Some species are venomous, either giving off a hemotoxin (blood poison) or a neurotoxin (nerve poison). The tongue is used in the sense of smell. Heat sensitive pits are a feature of some venomous snakes, these allows the snake to home in on the prey's body heat.
  • Order Crocodilia. Crocodiles evolved from the diapsid lineage. These organism are found in most of the warm regions of the world. They spend most of their time in the water. They contain a 4 chambered heart and are the closest living relative of the dinosaur.