Most bilaterian phyla, including arthropods, molluscs and chordates, have a two-opening gut tube with a mouth at one end and an anus at the other.

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.

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.

[5] 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.

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.

A fringe of tentacles thrusts food into the cavity and it can gape widely enough to accommodate large prey items.

[7] Annelids have simple tube-like guts, and the possession of an anus allows them to separate the digestion of their foodstuffs from the absorption of the nutrients.

These include mandibles, maxillae and labium and can be modified into suitable appendages for chewing, cutting, piercing, sponging and sucking.

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.

[14] They typically have many small hinged pedicellate teeth, the bases of which are attached to the jaws, while the crowns break off at intervals and are replaced.

Lacking teeth that are suitable for efficiently chewing of their food, turtles often have gastroliths in their stomach to further grind the plant material.

[21] 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, and upper and lower teeth.

[23] Some mammals rely on panting for thermoregulation as it increases evaporation of water across the moist surfaces of the lungs, the tongue and mouth.

Varying the position of the tongue in relation to the other articulators or moving the lips restricts the airflow from the lungs in different ways and changes the mouth's resonating properties, producing a range of different sounds.