Editing Sea cucumber (section)

=== Locomotive organs ===
Like all echinoderms, sea cucumbers possess [[pentaradial symmetry]], with their bodies divided into five nearly identical parts around a central axis. However, because of their posture, they have secondarily evolved a degree of bilateral symmetry. For example, because one side of the body is typically pressed against the substratum, and the other is not, there is usually some difference between the two surfaces (except for [[Apodida]]). Like [[sea urchin]]s, most sea cucumbers have five strip-like ambulacral areas running along the length of the body from the mouth to the anus. The three on the lower surface have numerous [[tube feet]], often with suckers, that allow the animal to crawl along; they are called ''trivium''. The two on the upper surface have under-developed or vestigial tube feet, and some species lack tube feet altogether; this face is called ''bivium''.<ref name=IZ/>

In some species, the ambulacral areas can no longer be distinguished, with tube feet spread over a much wider area of the body. Those of the order [[Apodida]] have no tube feet or ambulacral areas at all, and burrow through sediment with muscular contractions of their body similar to that of worms, however five radial lines are generally still obvious along their body.<ref name=IZ/>

Even in those sea cucumbers that lack regular tube feet, those that are immediately around the mouth are always present. These are highly modified into retractile [[tentacle]]s, much larger than the locomotive tube feet.  Depending on the species, sea cucumbers have between 10 and 30 such tentacles and these can have a wide variety of shapes depending on the diet of the animal and other conditions.<ref name=IZ/>

Many sea cucumbers have papillae, conical fleshy projections of the body wall with sensory tube feet at their apices.<ref>{{cite web|url=http://species-identification.org/species.php?species_group=nasc&selected=definitie&menuentry=woordenlijst&record=PapillaMarine|title=Marine Species Identification Portal : North Australian Sea Cucumbers : Glossary : PapillaMarine|access-date=12 June 2015}}</ref> These can even evolve into long antennae-like structures, especially on the abyssal genus ''[[Scotoplanes]]''.