Anus vs Cloaca
Not many people are aware of the fact that reptiles, birds and fish have a single opening for excretion of waste material as well as urine. This is in sharp contrast to mammals (including human beings) that have separate openings for excretion of urine and waste material. The opening in mammals that is a part of the digestive system and used for excretion of waste material is called anus whereas the opening in reptiles, birds and fish that is used for excreting both urine and feces is called cloaca. There are differences between anus and cloaca that will be talked about in this article.
The word cloaca comes from Latin word meaning sewer and it literally performs a function similar to sewer in vertebrates. It is a common chamber and outlet into which intestinal, urinary and genital tracts open. Cloaca is found in amphibians, reptiles, birds, fish and monotremes. However, cloaca is not found in placental mammals and most of the bony fishes. There are some species that have within this cloaca an accessory organ (penis) that males use to inject sperms into female cloaca. This type of structure is found in many birds and reptiles. Birds mate by joining their cloacas in a kiss and muscular contractions transfer sperms from male into female. Thus it is clear that cloaca is used for excretion as well as reproduction. This is also the opening that is used for laying eggs.
Anus on the other hand is simply an opening at the distal end of the alimentary canal. It is a passage for undigested waste only. Anus is used by mammals to excrete semi solid matter that cannot be digested. It was an important part of the evolutionary process that lead to the development of multicellular organisms in sharp contrast to single celled organisms.
Anus vs Cloaca
• Anus and cloaca perform the same function of excretion of waste
• Anus as an opening at the end of the digestive tract is found in mammals whereas cloaca is an opening used for excretion of feces as well as urine in reptiles, birds and fish
• Cloaca is also used for mating and laying of eggs by some species.