Editing Sea cucumber (section)

===Body plan===
The body of a holothuroid is roughly cylindrical. It is radially symmetrical along its longitudinal axis, and has weak bilateral symmetry transversely with a dorsal and a ventral surface. As in other [[echinozoa]]ns, there are five [[Ambulacral|ambulacra]] separated by five ambulacral grooves, the interambulacra. The ambulacral grooves bear four rows of tube feet but these are diminished in size or absent in some holothuroids, especially on the dorsal surface. The two dorsal ambulacra make up the bivium while the three ventral ones are known as the trivium.<ref name=Lander>[http://webs.lander.edu/rsfox/invertebrates/cucumaria.html ''Cucumaria frondosa''] {{webarchive|url=https://web.archive.org/web/20110903211033/http://webs.lander.edu/rsfox/invertebrates/cucumaria.html |date=2011-09-03 }} Invertebrate anatomy online. ed2012-02-12.</ref>

At the anterior end, the mouth is surrounded by a ring of tentacles which are usually retractable into the mouth. These are called the primary tentacles and were present in the common ancestor of echinoderms, but have been lost in all the other classes of the phylum,<ref>{{cite journal | url=https://link.springer.com/article/10.1134/S0031030122080020 | doi=10.1134/S0031030122080020 | title=Origin of Echinodermata | date=2022 | last1=Ezhova | first1=O. V. | last2=Malakhov | first2=V. V. | journal=Paleontological Journal | volume=56 | issue=8 | pages=938–973 | bibcode=2022PalJ...56..938E | url-access=subscription }}</ref> and may be simple, branched or arborescent. They are known as the introvert and posterior to them there is an internal ring of large calcareous ossicles. Attached to this are five bands of muscle running internally longitudinally along the ambulacra. There are also circular muscles, contraction of which cause the animal to elongate and the introvert to extend. Anterior to the ossicles lie further muscles, contraction of which cause the introvert to retract.<ref name=Lander/>

The body wall consists of an epidermis and a dermis and contains smaller calcareous ossicles, the types of which are characteristics which help to identify different species. Inside the body wall is the coelom which is divided by three longitudinal [[Mesentery (zoology)|mesenteries]] which surround and support the internal organs.<ref name=Lander/>