reptiles'). Despite their prehistoric look, crocodiles are among the more biologically complex reptiles. Unlike other reptiles, a crocodile has a cerebral cortex and a four-chambered heart. Crocodilians also have the functional equivalent of a diaphragm by incorporating muscles used for aquatic locomotion into respiration. Salt glands are present in the tongues of crocodiles and they have a pore opening on the surface of the tongue, a trait that separates them from alligators. Salt glands are dysfunctional in Alligatoridae. Their function appears to be similar to that of salt glands in marine turtles. Crocodiles do not have sweat glands and release heat through their mouths. They often sleep with their mouths open and may pant like a dog. Four species of freshwater crocodile climb trees to bask in areas lacking a shoreline. Senses Crocodiles have acute senses, an evolutionary advantage that makes them successful predators. The eyes, ears and nostrils are located on top of the head, allowing the crocodile to lie low in the water, almost totally submerged and hidden from prey. Vision Crocodiles have very good night vision, and are mostly nocturnal hunters. They use the disadvantage of most prey animals' poor nocturnal vision to their advantage. The light receptors in crocodilians' eyes include cones and numerous rods, so it is assumed all crocodilians can see colours. Crocodiles have vertical-slit shaped pupils, similar to those of domestic cats. One explanation for the evolution of slit pupils is that they exclude light more effectively than a circular pupil, helping to protect the eyes during daylight. On the rear wall of the eye is a tapetum lucidum, which reflects incoming light back onto the retina, thus utilizing the small amount of light available at night to best advantage. In addition to the protection of the upper and lower eyelids, crocodiles have a nictitating membrane (sometimes called a "third eye-lid") that can be drawn over the eye from the inner corner while the lids are open. The eyeball surface is thus protected under the water while a certain degree of vision is still possible. Olfaction Crocodilian sense of smell is also very well developed, aiding them to detect prey or animal carcasses that are either on land or in water, from far away. It is possible that crocodiles use olfaction in the egg prior to hatching. Chemoreception in crocodiles is especially interesting because they hunt in both terrestrial and aquatic surroundings. Crocodiles