Editing Flatworm (section)

===Turbellaria===

{{main |Turbellaria}}

[[File:Pseudoceros dimidiatus.jpg|thumb |The turbellarian ''[[Pseudoceros dimidiatus]]'']]

[[File:Mating Pseudobiceros bedfordi.png|thumb |Two [[turbellaria]]ns are mating by [[penis fencing]]. Each has two penises, the white spikes on the undersides of their heads.]]

These have about 4,500 species,<ref name="RuppertBarnes2004Platyhelminthes"/> are mostly free-living, and range from {{convert|1|mm|in|2|abbr=on}} to {{convert|600|mm|in|abbr=on}} in length. Most are predators or scavengers, and terrestrial species are mostly nocturnal and live in shaded, humid locations, such as leaf litter or rotting wood. However, some are [[symbiosis|symbiotes]] of other animals, such as [[crustacea]]ns, and some are [[parasite]]s. Free-living turbellarians are mostly black, brown or gray, but some larger ones are brightly colored.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /> The [[Acoela]] and [[Nemertodermatida]] were traditionally regarded as turbellarians,<ref name="RuppertBarnes2004Platyhelminthes" /><ref name="JondeliusEtAl2002Nemertodermatida" /> but are now regarded as members of a separate phylum, the [[Acoelomorpha]],<ref name="Halanych2004NewViewOfAnimalPhylogeny" /><ref name="LarssonJondelius2008PhylogenyOfCatenulida" /> or as two separate phyla.<ref name="Wallberg2007EtAlDismissalOfAcoelomorpha" /> ''[[Xenoturbella]]'', a [[genus]] of very simple animals,<ref name="Westblad1949Xenoturbella" /> has also been reclassified as a separate phylum.<ref name="BourlatEtAl2003XenoturbellaPhylum" />

Some turbellarians have a simple [[pharynx]] lined with [[Cilium|cilia]] and generally feed by using cilia to sweep food particles and small prey into their mouths, which are usually in the middle of their undersides. Most other turbellarians have a pharynx that is eversible (can be extended by being turned inside-out), and the mouths of different species can be anywhere along the underside.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /> The freshwater species ''[[Microstomum caudatum]]'' can open its mouth almost as wide as its body is long, to swallow prey about as large as itself.<ref name="RuppertBarnes2004Platyhelminthes" /> Predatory species in suborder [[Kalyptorhynchia]] often have a muscular pharynx equipped with hooks or teeth used for seizing prey.<ref>{{cite journal | pmc=4612338 | date=2015 | author1=Smith JP III | last2=Litvaitis | first2=M. K. | last3=Gobert | first3=S. | last4=Uyeno | first4=T. | last5=Artois | first5=T. | title=Evolution and Functional Morphology of the Proboscis in Kalyptorhynchia (Platyhelminthes) | journal=Integrative and Comparative Biology | volume=55 | issue=2 | pages=205–216 | doi=10.1093/icb/icv056 | pmid=26002347 }}</ref>

Most turbellarians have pigment-cup [[ocelli]] ("little eyes"); one pair in most species, but two or even three pairs in others. A few large species have many eyes in clusters over the brain, mounted on tentacles, or spaced uniformly around the edge of the body. The ocelli can only distinguish the direction from which light is coming to enable the animals to avoid it. A few groups have [[statocyst]]s - fluid-filled chambers containing a small, solid particle or, in a few groups, two. These statocysts are thought to function as balance and acceleration sensors, as they perform the same way in [[cnidaria]]n [[medusae]] and in [[ctenophore]]s. However, turbellarian statocysts have no sensory cilia, so the way they sense the movements and positions of solid particles is unknown. On the other hand, most have ciliated touch-sensor cells scattered over their bodies, especially on tentacles and around the edges. Specialized cells in pits or grooves on the head are most likely smell sensors.<ref name="RuppertBarnes2004Platyhelminthes" />

[[Planarian]]s, a subgroup of seriates, are famous for their ability to regenerate if divided by cuts across their bodies. Experiments show that (in fragments that do not already have a head) a new head grows most quickly on those fragments which were originally located closest to the original head. This suggests the growth of a head is controlled by a chemical whose concentration diminishes throughout the organism, from head to tail. Many turbellarians [[cloning|clone]] themselves by transverse or longitudinal division, whilst others, reproduce by [[budding]].<ref name="RuppertBarnes2004Platyhelminthes" />

The vast majority of turbellarians are [[hermaphrodite]]s (they have both female and male reproductive cells) which fertilize eggs [[internal fertilization|internally]] by [[copulation (zoology)|copulation]].<ref name="RuppertBarnes2004Platyhelminthes" /> Some of the larger aquatic species mate by [[penis fencing]] – a duel in which each tries to impregnate the other, and the loser adopts the female role of developing the eggs.<ref>{{cite web |url=https://www.pbs.org/kcet/shapeoflife/episodes/hunt_explo2.html |access-date=2008-12-21 |title=Fighting to mate: flatworm penis fencing |author=Newman, Leslie |publisher=[[Public Broadcasting Service|PBS]] |archive-date=2013-11-14 |archive-url=https://web.archive.org/web/20131114002014/http://www.pbs.org/kcet/shapeoflife/episodes/hunt_explo2.html |url-status=live }}</ref> In most species, "miniature adults" emerge when the eggs hatch, but a few large species produce [[plankton]]-like [[larva]]e.<ref name="RuppertBarnes2004Platyhelminthes" />
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