, "the lizard of the (Nile) river". There are several variant Greek forms of the word attested, including the later form () found cited in many English reference works. In the Koine Greek of Roman times, and would have been pronounced identically, and either or both may be the source of the Latinized form used by the ancient Romans. It has been suggested, but it is not certain that the word or is a compound of ('pebbles'), and ('worm'), although is only attested as a colloquial term for 'penis'. It is ascribed to Herodotus, and supposedly describes the basking habits of the Egyptian crocodile. The form is attested in Medieval Latin. It is not clear whether this is a medieval corruption or derives from alternative Greco-Latin forms (late Greek and are attested). A (further) corrupted form is found in Old French and was borrowed into Middle English as . The Modern English form crocodile was adapted directly from the Classical Latin in the 16th century, replacing the earlier form. The use of -y- in the scientific name Crocodylus (and forms derived from it) is a corruption introduced by Laurenti (1768). Species A total of 18 extant species have been recognized. Further genetic study is needed for the confirmation of proposed species under the genus Osteolaemus. Characteristics A crocodile's physical traits allow it to be a successful predator. Its external morphology is a sign of its aquatic and predatory lifestyle. Its streamlined body enables it to swim swiftly; it also tucks its feet to the side while swimming, making it faster by decreasing water resistance. Crocodiles have webbed feet which, though not used to propel them through the water, allow them to make fast turns and sudden moves in the water or initiate swimming. Webbed feet are an advantage in shallow water, where the animals sometimes move around by walking. Crocodiles have a palatal flap, a rigid tissue at the back of the mouth that blocks the entry of water. The palate has a special path from the nostril to the glottis that bypasses the mouth. The nostrils are closed during submergence. Like other archosaurs, crocodilians are diapsid, although their post-temporal fenestrae are reduced. The walls of the braincase are bony but lack supratemporal and postfrontal bones. Their tongues are not free, but held in place by a membrane that limits movement; as a result, crocodiles are unable to stick out