Editing Flatworm (section)

===Features common to all subgroups===

The lack of circulatory and respiratory organs limits platyhelminths to sizes and shapes that enable [[oxygen]] to reach and [[carbon dioxide]] to leave all parts of their bodies by simple [[diffusion]]. Hence, many are microscopic, and the large species have flat ribbon-like or leaf-like shapes. Because there is no circulatory system which can transport nutrients around, the guts of large species have many branches, allowing the nutrients to diffuse to all parts of the body.<ref name="Barnes1998DiversityOfLifePlatyhelminthes" /> Respiration through the whole surface of the body makes them vulnerable to fluid loss, and restricts them to environments where [[dehydration]] is unlikely: sea and freshwater, moist terrestrial environments such as [[leaf litter]] or between grains of soil, and as [[parasite]]s within other animals.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" />

The space between the skin and gut is filled with [[mesenchyme]], also known as [[Parenchyma#Flatworms|parenchyma]], a [[connective tissue]] made of [[cell (biology)|cells]] and reinforced by [[collagen]] fibers that act as a type of [[skeleton]], providing attachment points for [[muscle]]s. The mesenchyme contains all the internal organs and allows the passage of oxygen, nutrients and waste products. It consists of two main types of cell: fixed cells, some of which have fluid-filled [[vacuole]]s; and [[stem cell]]s, which can transform into any other type of cell, and are used in regenerating tissues after injury or [[asexual reproduction]].<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" />

Most platyhelminths have no [[anus]] and regurgitate undigested material through the mouth. The genus ''[[Paracatenula]]'', whose members include tiny flatworms living in symbiosis with bacteria, is even missing a mouth and a gut.<ref>{{Cite web |url=https://www.iflscience.com/this-seabed-flatworm-got-rid-of-its-mouth-and-anus-replacing-its-entire-digestive-system-with-bacteria-52075 |title=This Seabed Flatworm Got Rid Of Its Mouth And Anus, Replacing Its Entire Digestive System With Bacteria |date=8 April 2019 |access-date=2022-12-11 |archive-date=2022-12-11 |archive-url=https://web.archive.org/web/20221211200016/https://www.iflscience.com/this-seabed-flatworm-got-rid-of-its-mouth-and-anus-replacing-its-entire-digestive-system-with-bacteria-52075 |url-status=live }}</ref> However, some long species have an anus and some with complex, branched guts have more than one anus, since excretion only through the mouth would be difficult for them.<ref name="RuppertBarnes2004Platyhelminthes" /> The gut is lined with a single layer of [[endoderm]]al cells that absorb and digest food. Some species break up and soften food first by secreting [[enzyme]]s in the gut or [[pharynx]] (throat).<ref name="WalkerAnderson2001PlatyhelminthesInAnderson"/>

All animals need to keep the [[Concentration (chemistry)|concentration]] of dissolved substances in their body fluids at a fairly constant level. Internal parasites and free-living marine animals live in environments with high concentrations of dissolved material, and generally let their tissues have the same level of concentration as the environment, while freshwater animals need to prevent their body fluids from becoming too dilute. Despite this difference in environments, most platyhelminths use the same system to [[osmoregulation|control the concentration]] of their body fluids. [[Flame cell]]s, so called because the beating of their [[flagella]] looks like a flickering candle flame, extract from the mesenchyme water that contains wastes and some reusable material, and drive it into networks of tube cells which are lined with flagella and [[microvilli]]. The tube cells' flagella drive the water towards exits called [[nephridiopore]]s, while their microvilli reabsorb reusable materials and as much water as is needed to keep the body fluids at the right concentration. These combinations of flame cells and tube cells are called [[protonephridia]].<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /><ref name="RuppertBarnes2004Bilateria" />

In all platyhelminths, the [[nervous system]] is concentrated at the head end. Other platyhelminths have rings of [[Ganglion|ganglia]] in the head and main nerve trunks running along their bodies.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /><ref name="RuppertBarnes2004Platyhelminthes" />