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The anus is an opening at the opposite end of an animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
's digestive tract from the mouth
Mouth

The mouth, buccal cavity, or oral cavity is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva....
. Its function is to expel feces
Feces

Feces, faeces, or f?ces is a waste product from an animal's gastrointestinal tract expelled through the anus during defecation....
, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as bones
Coprolite

A coprolite is fossilized animal dung. Coprolites are classified as Trace fossil as opposed to body fossils, as they give evidence for the animal's behaviour rather than morphology....
; food material after all the nutrients have been extracted, for example cellulose
Cellulose

File:Cellulose Sessel.svgCellulose is an organic compound with the chemical formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand ? linked D-glucose units....
 or lignin
Lignin

Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae....
; ingested matter which would be toxic if it remained in the digestive tract; and dead or excess gut bacteria and other endosymbiont
Endosymbiont

An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis . Examples are nitrogen-fixing bacterium which live in root nodules on legume roots, single-celled algae inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to about 10%?15% of in...
s.

Amphibian
Amphibian

Amphibians , such as frogs, toads, salamanders, newts and caecilians, are cold-blooded animals that metamorphose from a juvenile, water-breathing form to an adult, air-breathing form....
s, reptile
Reptile

Reptiles, or members of the class Reptilia, are air-breathing, cold-blooded vertebrates that have skin covered in scale as opposed to hair or feathers....
s, and bird
Bird

Birds are wing, Bipedalismal, endothermic , vertebrate animals that lay egg . There are around 10,000 living species, making them the most numerous tetrapod vertebrates....
s use the same orifice for excreting liquid and solid wastes, and for copulation and egg
Egg (biology)

In most birds and reptiles, an egg is the zygote, resulting from fertilization of the ovum. To enable incubation the egg is usually kept within a favourable temperature range as it nourishes and protects the growing embryo....
-laying; this orifice is known as the cloaca
Cloaca

In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the alimentary tract and urinary tract of certain animal species....
.






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Anorectum
The anus is an opening at the opposite end of an animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
's digestive tract from the mouth
Mouth

The mouth, buccal cavity, or oral cavity is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva....
. Its function is to expel feces
Feces

Feces, faeces, or f?ces is a waste product from an animal's gastrointestinal tract expelled through the anus during defecation....
, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as bones
Coprolite

A coprolite is fossilized animal dung. Coprolites are classified as Trace fossil as opposed to body fossils, as they give evidence for the animal's behaviour rather than morphology....
; food material after all the nutrients have been extracted, for example cellulose
Cellulose

File:Cellulose Sessel.svgCellulose is an organic compound with the chemical formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand ? linked D-glucose units....
 or lignin
Lignin

Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae....
; ingested matter which would be toxic if it remained in the digestive tract; and dead or excess gut bacteria and other endosymbiont
Endosymbiont

An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis . Examples are nitrogen-fixing bacterium which live in root nodules on legume roots, single-celled algae inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to about 10%?15% of in...
s.

Amphibian
Amphibian

Amphibians , such as frogs, toads, salamanders, newts and caecilians, are cold-blooded animals that metamorphose from a juvenile, water-breathing form to an adult, air-breathing form....
s, reptile
Reptile

Reptiles, or members of the class Reptilia, are air-breathing, cold-blooded vertebrates that have skin covered in scale as opposed to hair or feathers....
s, and bird
Bird

Birds are wing, Bipedalismal, endothermic , vertebrate animals that lay egg . There are around 10,000 living species, making them the most numerous tetrapod vertebrates....
s use the same orifice for excreting liquid and solid wastes, and for copulation and egg
Egg (biology)

In most birds and reptiles, an egg is the zygote, resulting from fertilization of the ovum. To enable incubation the egg is usually kept within a favourable temperature range as it nourishes and protects the growing embryo....
-laying; this orifice is known as the cloaca
Cloaca

In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the alimentary tract and urinary tract of certain animal species....
. Monotreme
Monotreme

Monotremes are mammals that lay eggs instead of giving birth to live young like Marsupialias and Placentalia .They are conventionally treated as comprising a single order Monotremata, though a recent classification proposes to divide them into the orders Platypoda and Tachyglossa ....
 mammals also have a cloaca, which is thought to be a feature inherited from the earliest amniote
Amniote

The amniotes are a group of tetrapod vertebrates that have a terrestrially adapted egg. They include the Synapsida and Sauropsida . Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes....
s via the therapsids. Marsupial
Marsupial

Marsupials are an infraclass of mammals, characterized by a distinctive Pouch , in which females carry their young through early infancy....
s have two nether orifices: one for excreting both solids and liquids; the other for reproduction, which appears as a vagina
Vagina

The vagina is a fibromuscular cylinder tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles....
 in females and a penis
Penis

The penis is an external sex organ of certain biologically male organisms, in both vertebrates and invertebrates.The penis is a reproductive organ, technically an intromittent organ, and for Eutheria, additionally serves as the external organ of urination....
 in males. Female placental mammals have completely separate orifices for defecation
Defecation

Defecation is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material from the digestive tract via the anus....
, urination
Urination

Urination, also known as micturition, voiding, and, more rarely, emiction, is the process of disposing urine from the urinary bladder through the urethra to the outside of the body....
, and reproduction; males have one opening for defecation and another for both urination and reproduction, although the channels flowing to that orifice are almost completely separate.

The development of the anus was an important stage in the evolution
Evolution

In biology, evolution is change in the heritability trait of a population of organisms from one generation to the next. These changes are caused by a combination of three main processes: variation, reproduction, and selection....
 of multicellular animals. In fact it appears to have happened at least twice, following different paths in protostome
Protostome

Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phylum, they make up the Bilateria, mostly comprising animals with symmetry #Bilateral symmetry and triploblastic germ layers....
s and deuterostome
Deuterostome

Deuterostomes are a superphylum of animals. They are a taxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes....
s. This accompanied or facilitated other important evolutionary developments: the bilaterian body plan
Body plan

A body plan, or bauplan, is essentially the blueprint for the way the body of an organism is laid out. An organism's symmetry , its number of body segments and number of Limb are all aspects of its body plan....
; the coelom
Coelom

The coelom is a fluid filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages....
, an internal cavity that provided space for a circulatory system
Circulatory system

The circulatory system is an organ that moves nutrients, gases, and wastes to and from cells to help fight diseases and help stabilize body temperature and pH to maintain homeostasis....
 and, in some animals, formed a hydrostatic skeleton
Hydrostatic skeleton

A hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles....
 which enables worm-like animals to burrow; metamerism
Metamerism

Metamerism has at least three meanings:*Metamerism is the property of having repeated segmentation , as in annelids; a concern of zoology and developmental biology...
, in which the body was built of repeated "modules" which could later specialize, for example the heads of most arthropods are composed of fused, specialized segments.

Development

In animals at least as complex as an earthworm
Earthworm

Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. The earthworm is the most known worm in America, and other countries....
, the embryo
Embryo

An embryo is a multicellular organism ploidy eukaryote in its earliest stage of development, from the time of first cell division until birth, Egg , or germination....
 forms a dent on one side, the blastopore
Blastopore

A blastopore is an opening into the archenteron during the embryonic stages of an organism. The Embryological origins of the mouth and anus protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore....
, which deepens to become the archenteron
Archenteron

The primitive gut that forms during gastrulation in the developing blastula is known as the archenteron. It develops into the digestive tract of an animal....
, the first phase in the growth of the gut. In deuterostome
Deuterostome

Deuterostomes are a superphylum of animals. They are a taxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes....
s, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. The protostome
Protostome

Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phylum, they make up the Bilateria, mostly comprising animals with symmetry #Bilateral symmetry and triploblastic germ layers....
s were so named because it used to be thought that in their embryos the dent formed the mouth while the anus was formed later, at the opening made by the other end of the gut. More recent research, however, shows that in protostomes the edges of the dent close up in the middle, leaving openings at the ends which become the mouth and anus.

Physiology

Flow of substance through the anus is typically controlled by the anal sphincter
Sphincter

A sphincter is a structure, usually a circular muscle, that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning....
 muscle.