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.
The horny structures covering the mouth of a bird compose its beak or bill.
The human mouth is covered by an upper and lower lip. They play an important role in talking, facial expression, kissing, drinking (especially with a straw), oral sex, smoking. 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.
According to etiquette the mouth is kept closed during chewing.