Editing Frog (section)

===Digestion and excretion===
Frogs have maxillary teeth along their upper jaw which are used to hold food before it is swallowed. These teeth are very weak, and cannot be used to chew or catch and harm agile prey. Instead, the frog uses its sticky, cleft tongue to catch insects and other small moving prey. The tongue normally lies coiled in the mouth, free at the back and attached to the mandible at the front. It can be shot out and retracted at great speed.<ref name=Exploratorium/> In amphibians there are salvary glands on the tongue, which in frogs produce what is called a two-phase viscoelastic fluid. When exposed to pressure, like when the tongue is wrapping around a prey, it becomes runny and covers the prey's body. As the pressure drops, it returns to a thick and elastic state, which gives the tongue an extra grip.<ref>[https://stao.ca/the-secret-to-the-stickiness-of-frog-spit/ The Secret to the Stickiness of Frog Spit | STAO]</ref> Some frogs have no tongue and just stuff food into their mouths with their hands.<ref name=Exploratorium/> The African bullfrog (''[[Pyxicephalus]]''), which preys on relatively large animals such as mice and other frogs, has cone shaped bony projections called odontoid processes at the front of the lower jaw which function like teeth.<ref name="Pou92"/> The eyes assist in the swallowing of food as they can be retracted through holes in the skull and help push food down the throat.<ref name=Exploratorium/><ref>{{cite journal |last1=Levine |first1=R. P. |last2=Monroy |first2=J. A. |last3=Brainerd|first3=E. L. |title=Contribution of eye retraction to swallowing performance in the northern leopard frog, ''Rana pipiens'' |doi=10.1242/jeb.00885|pmid=15010487 |date=March 15, 2004|pages=1361–1368 |issue=Pt 8|volume=207 |journal=Journal of Experimental Biology |doi-access=free |bibcode=2004JExpB.207.1361L }}</ref>

The food then moves through the oesophagus into the stomach where digestive enzymes are added and it is churned up. It then proceeds to the small intestine (duodenum and ileum) where most digestion occurs. Pancreatic juice from the pancreas, and bile, produced by the liver and stored in the gallbladder, are secreted into the small intestine, where the fluids digest the food and the nutrients are absorbed. The food residue passes into the large intestine where excess water is removed and the wastes are passed out through the [[cloaca]].<ref>{{cite web |url=http://www.tutorvista.com/biology/frog-digestive-system-diagram |title=Frog Digestive System |year=2010 |publisher=TutorVista.com |access-date=August 4, 2012|archive-url=https://web.archive.org/web/20100603080405/http://www.tutorvista.com/biology/frog-digestive-system-diagram|archive-date=June 3, 2010|url-status=dead}}</ref>

Although adapted to terrestrial life, frogs resemble freshwater fish in their inability to conserve body water effectively. When they are on land, much water is lost by evaporation from the skin. The excretory system is similar to that of mammals and there are two [[kidney]]s that remove nitrogenous products from the blood. Frogs produce large quantities of dilute urine in order to flush out toxic products from the kidney tubules.<ref name=Doritexcretion>{{cite book |title=Zoology |url=https://archive.org/details/zoology0000dori |url-access=registration |last=Dorit |first=R. L. |author2=Walker, W. F.|author3=Barnes, R. D. |year=1991 |publisher=Saunders College Publishing  |isbn=978-0-03-030504-7 |page=[https://archive.org/details/zoology0000dori/page/849 849] }}</ref> The nitrogen is excreted as [[ammonia]] by tadpoles and aquatic frogs but mainly as [[urea]], a less toxic product, by most terrestrial adults. A few species of tree frog with little access to water excrete the even less toxic [[uric acid]].<ref name=Doritexcretion/> The urine passes along paired [[ureter]]s to the [[urinary bladder]] from which it is vented periodically into the cloaca. All bodily wastes exit the body through the cloaca which terminates in a cloacal vent.<ref name=TutorVista>{{cite web |url=http://www.tutorvista.com/content/biology/biology-iii/animal-morphology/respiratory-excretory-nervous-reproductive-system-frog.php |title=Frog's internal systems |year=2010 |publisher=TutorVista.com |access-date=June 4, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20080121101351/http://www.tutorvista.com/content/biology/biology-iii/animal-morphology/respiratory-excretory-nervous-reproductive-system-frog.php |archive-date=January 21, 2008}}</ref>