Editing Excretory system (section)

==== Skin ====
[[Sweat gland]]s in the skin secrete a fluid waste called sweat or [[perspiration]]; however, its primary functions are temperature control and pheromone release. Therefore, its role as a part of the excretory system is minimal. Sweating also maintains the level of salt in the body.

Mammals excrete sweat through sweat glands in the [[skin]] throughout the body. The sweat, helped by salt, evaporates and helps to keep the body cool when it is warm. In amphibians, the [[lung]]s are very simple, and they lack the necessary means to the exhale like other [[tetrapod]]s can. The moist, scale-less skin is therefore essential in helping to rid the blood of [[carbon dioxide]], and also allows for urea to be expelled through diffusion when submerged.<ref name=Breathing>{{cite journal|last=Janis|first=C.M.|author2=Keller, J.C. |title=Modes of ventilation in early tetrapods: Costal aspiration as a key feature of amniotes|journal=Acta Palaeontologica Polonica|year=2001|volume=46|issue=2|pages=137–170|url=http://www.app.pan.pl/archive/published/app46/app46-137.pdf|access-date=11 May 2012}}</ref>

In small-bodied marine invertebrates, the skin is the most important excretory organ. That is particularly true for [[acoelomate]] groups like [[cnidaria]]ns, [[flatworm]]s and [[nemertea]]ns, who have no body cavities and hence no body fluid that can be drained or purified by nephrons, which is the reason acoelomate animals are thread-like (nemertans), flat (flatworms) or only consist of a thin layer of cells around a gelatinous non-cellular interior (cnidarians).<ref name=Dorit>{{cite book |title=Zoology |url=https://archive.org/details/zoology0000dori |url-access=registration |last1=Dorit |first1=R. L. |last2=Walker |first2=W. F. |last3=Barnes |first3=R. D. |year=1991 |publisher=Saunders College Publishing |isbn=978-0-03-030504-7 }}</ref>