Crocodilians are the most advanced surviving reptiles; many of their features are more similar to mammals or birds than to other reptiles. There are three families of Crocodilia, with 23 species total. All crocodilians have a similar body shape, with a head held horizontally in front of the body, four legs which project from the sides, heavy scales which function as armor, and a heavy muscular tail. Their front feet have five separate toes and their rear feet have four partially-webbed toes. Their eyes are on the top of their head, close together to allow for binocular vision (the field of vision of the two eyes intersects, to provide more accurate depth perception in front of the animal). The nostrils are crescent-shaped and valvular, and set at the end of the snout, which allows breathing even when the animal is almost entirely submerged. Crocodilians range in size from Cuvier's dwarf caiman, which only grows to about 1.5 meters (five feet) long, to the Indopacific crocodile, which grows to 7 meters (23 feet) long. Male crocodilians are larger than females.