Amphibians,reptiles andbirds use a similar orifice (known as thecloaca) for excreting liquid and solid wastes, forcopulation andegg-laying.Monotreme mammals also have a cloaca, which is thought to be a feature inherited from the earliestamniotes.Marsupials have a single orifice for excreting both solids and liquids and, in females, a separatevagina for reproduction. Femaleplacental mammals have completely separate orifices fordefecation,urination, and reproduction; males have one opening for defecation andanother 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 inprotostomes anddeuterostomes. This accompanied or facilitated other important evolutionary developments: thebilaterian body plan, thecoelom, andmetamerism, in which the body was built of repeated "modules" which could later specialize, such as the heads of mostarthropods, which are composed of fused, specialized segments.
Incomb jellies, there are species with one and sometimes two permanent anuses, species like thewarty comb jelly grows an anus, which then disappear when it is no longer needed.[4]
In animals at least as complex as anearthworm, theembryo forms a dent on one side, theblastopore, which deepens to become thearchenteron, the first phase in the growth of thegut. 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.[5]
