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The mouth is the opening through which an animal takes in food. It is usually located in the head, but not always; the mouth of a planarium is in the middle of its belly.

Most animals have a complete digestive system, with a mouth at one end and an anus at the other. Which end forms first in ontogeny is a criterion used to classify animals into protostome and deuterostome.

Some animals, such as the cnidarians and the planaria, have no anus. They expel waste through the mouth, or as do brachiopods, which have very efficient digestive systems, roll it around in the stomach until it's finished.

Some animals, such as the tapeworm, don't even have a mouth. The tapeworm lives inside the digestive system, so it doesn't need its own digestive system.

In many animals, the mouth has mouthparts to chew food, hold food, or inject poison. In arthropods, they are external modified legs; in gnathostome vertebratePetromyzontidae ( lampreys) Placodermi extinct Chondrichthyes (cartilaginous fish) Acanthodii extinct Actinopterygii (ray-finned fish) Actinistia ( coelacanths) Dipnoi ( lungfish) Amphibia ( amphibians) Reptilia ( reptiles) Aves ( birds) Mammalia ( mammals, they are internal.

The horny structures covering the mouth of a bird compose its beakThe beak otherwise known as the bill is the only device a bird has for consuming food. Birds have no teeth so they must swallow their food whole. A bird's beak can vary in size and shape depending on the nature of their diet. Some birds such as the falcon or bill.

The human mouth is covered by an upper and lower lip. They play an important role in speechSpeech (n. an oral presentation by one person to a group (or sometimes just an individual); closely related terms include: Conversation (infomal speech by more than one person on a topic), Debate (formal communication between two groups holding opposing v, facial expressionA facial expression results from one or more motions or positions of the muscles of the face. They are closely associated with our emotions. Charles Darwin noted in his book The Expression of the Emotions in Man and Animal :. the young and the old of wide, kissing, drinkThe word drink is primarily a verb, meaning to ingest liquids, see Drinking. As a noun, it refers to the liquid thus ingested. It is often used in a narrower sense to refer to alcoholic beverages (as both a verb and a noun). It can also be used metaphoricing (especially with a straw), oral sex, smokingTobacco smoking is the act of smoking tobacco products, especially cigarettes and cigars. Tobacco smoking is considered a significant cause of human health problems, especially cancer and other disorders affecting the lungs. The practice of smoking tobacc. Infants are born with a sucking reflex, by which they instinctively know to suck for nourishment using their lips and jaw. Lips are often adorned with lipstick or lip gloss. The philtrum is the vertical groove in the upper lip, formed where the nasomedial and maxillary processes meet during embryo development. When these processes fail to fuse fully, a hare lip and/or Cleft palate can result.

According to etiquette the mouth is kept closed during chewing.

See also Tongue, Teeth, Lip piercing.