The anus (
Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
, 'ring' or 'circle') is an opening at the opposite end of an animal's
digestive tract from the mouth. Its function is to control the expulsion of
feces, the residual semi-solid waste that remains after food digestion, which, depending on the type of animal, includes: matter which the animal cannot digest, such as
bones;
[ Summary at ] food material after the nutrients have been extracted, for example
cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
or
lignin; ingested matter which would be toxic if it remained in the digestive tract; and dead or excess
gut bacteria
Gut microbiota, gut microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the ...
and other
endosymbionts.
Amphibians, reptiles, and birds use the same orifice (known as the
cloaca) for excreting liquid and solid wastes, for
copulation
Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetra ...
and
egg-laying.
Monotreme mammals also have a cloaca, which is thought to be a feature inherited from the earliest
amniotes via the
therapsids.
Marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in ...
s have a single orifice for excreting both solids and liquids and, in females, a separate
vagina for reproduction. Female
placental mammals have completely separate orifices for
defecation,
urination, 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 of multicellular animals. It appears to have happened at least twice, following different paths in
protostome
Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
s and
deuterostome
Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some ...
s. This accompanied or facilitated other important evolutionary developments: the
bilaterian body plan, the
coelom, and
metamerism, in which the body was built of repeated "modules" which could later specialize, such as the heads of most
arthropods, which are composed of fused, specialized segments.
Development
In animals at least as complex as an
earthworm
An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. T ...
, the
embryo forms a dent on one side, the
blastopore, which deepens to become the
archenteron, the first phase in the growth of the
gut. In deuterostomes, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. The protostomes were so named because it was thought that in their embryos the dent formed the mouth first (''
proto–'' meaning "first") and the anus was formed later at the opening made by the other end of the gut. Research from 2001 shows the edges of the dent close up in the middles of protosomes, leaving openings at the ends which become the mouths and anuses.
See also
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References
External links
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{{Authority control
Digestive system