In animal anatomy, the mouth, also known as the oral cavity, buccal cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on the outside by the lips and inside by the pharynx and containing in higher vertebrates the tongue and teeth. This cavity is also known as the buccal cavity, from the Latin bucca ("cheek").
Some animal phyla, including vertebrates, 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 protostomes and deuterostomes.
In the first multicellular animals, there was probably no mouth or gut and food particles were engulfed by the cells on the exterior surface by a process known as endocytosis. The particles became enclosed in vacuoles into which enzymes were secreted and digestion took place intracellularly. The digestive products were absorbed into the cytoplasm and diffused into other cells. This form of digestion is used nowadays by simple organisms such as Amoeba and Paramecium and also by sponges which, despite their large size, have no mouth or gut and capture their food by endocytosis.
The vast majority of other multicellular organisms have a mouth and a gut, the lining of which is continuous with the epithelial cells on the surface of the body.
In animals at least as complex as an earthworm, the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the formation of the gut. In deuterostomes, the blastopore becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. In the protostomes, it used to be thought that the blastopore formed the mouth (proto– meaning "first") while the anus formed later as an opening made by the other end of the gut. More recent research, however, shows that in protostomes the edges of the slit-like blastopore close up in the middle, leaving openings at both ends that become the mouth and anus.
Apart from sponges and placozoans, almost all animals have an internal gut cavity which is lined with gastrodermal cells. In less advanced invertebrates such as the sea anemone, the mouth also acts as an anus. Circular muscles around the mouth are able to relax or contract in order to open or close it. A fringe of tentacles thrusts food into the cavity and it can gape widely enough to accommodate large prey items. Food passes first into a pharynx and digestion occurs extracellularly in the gastrovascular cavity. Annelids have simple tube-like gets and the possession of an anus allows them to separate the digestion of their foodstuffs from the absorption of the nutrients. Many molluscs have a radula which is used to scrape microscopic particles off surfaces. In invertebrates with hard exoskeletons, various mouthparts may be involved in feeding behaviour. Insects have a range of mouthparts suited to their mode of feeding. These include mandibles, maxillae and labium and can be modified into suitable appendages for chewing, cutting, piercing, sponging and sucking. Decapods have six pairs of mouth appendages, one pair of mandibles, two pairs of maxillae and three of maxillipeds. Sea urchins have a set of five sharp calcareous plates which are used as jaws and are known as Aristotle's lantern.
In vertebrates, the first part of the digestive system is the buccal cavity, commonly known as the mouth. The buccal cavity of a fish is separated from the opercular cavity by the gills. Water flows in through the mouth, passes over the gills and exits via the operculum or gill slits. Nearly all fish have jaws and may seize food with them but most feed by opening their jaws, expanding their pharynx and sucking in food items. The food may be held or chewed by teeth located in the jaws, on the roof of the mouth, on the pharynx or on the gill arches.
Nearly all amphibians are carnivorous as adults.
The mouths of reptiles are largely similar to those of mammals.
Birds do not have teeth, relying instead on other means of gripping and macerating their food.
In mammals the buccal cavity is typically roofed by the hard and soft palates, floored by the tongue and surrounded by the cheeks, salivary glands, upper and lower teeth. The upper teeth are embedded in the upper jaw and the lower teeth in the lower jaw, which articulates with the temporal bones of the skull. The lips are soft and fleshy folds which shape the entrance into the mouth. The buccal cavity empties through the pharynx into the oesophagus.
Other functions of the mouth
Crocodilians living in the tropics can gape with their mouths to provide cooling by evaporation from the mouth lining. Some mammals rely on panting for thermoregulation as it increases evaporation of water across the moist surfaces of the lungs, the tongue and mouth. Birds also avoid overheating by gular fluttering, flapping the wings near the gular (throat) skin, similar to panting in mammals.
Various animals use their mouths in threat displays.
A number of species of bird use a gaping, open beak in their fear and threat displays.
Mouths are also used as part of the mechanism for producing sounds for communication.