Flatworm

Flatworm Temporal range: 270–0  Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N Possible Cambrian, Ordovician and Devonian records[2][3]
In a spiral, starting from top left: trematodes), Pseudobiceros hancockanus (Turbellaria )
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
(unranked):	Protostomia
(unranked):	Spiralia
Clade:	Rouphozoa
Phylum:	Platyhelminthes Claus, 1887
Classes
Traditional: Turbellaria Trematoda Monogenea Cestoda Phylogenetic: Catenulida Rhabditophora
Synonyms
Plathelminthes Schneider, 1873[4]

The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the

Distinguishing features

Platyhelminthes are

The space between the skin and gut is filled with mesenchyme, also known as parenchyma, a connective tissue made of cells and reinforced by collagen fibers that act as a type of skeleton, providing attachment points for muscles. 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 vacuoles; and stem cells, which can transform into any other type of cell, and are used in regenerating tissues after injury or asexual reproduction.^[5]

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.^[16] 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.^[13] The gut is lined with a single layer of endodermal cells that absorb and digest food. Some species break up and soften food first by secreting enzymes in the gut or pharynx (throat).^[5]

All animals need to keep the

In all platyhelminths, the nervous system is concentrated at the head end. Other platyhelminths have rings of ganglia in the head and main nerve trunks running along their bodies.^[5][13]

Major subgroups

Early classification divided the flatworms in four groups: Turbellaria, Trematoda, Monogenea and Cestoda. This classification had long been recognized to be artificial, and in 1985, Ehlers

These have about 4,500 species,

Some turbellarians have a simple pharynx lined with 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.^[5] 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.^[13] Predatory species in suborder Kalyptorhynchia often have a muscular pharynx equipped with hooks or teeth used for seizing prey.^[25]

Most turbellarians have pigment-cup

Planarians, 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 clone themselves by transverse or longitudinal division, whilst others, reproduce by budding.^[13]

The vast majority of turbellarians are hermaphrodites (they have both female and male reproductive cells) which fertilize eggs internally by copulation.^[13] 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.^[26] In most species, "miniature adults" emerge when the eggs hatch, but a few large species produce plankton-like larvae.^[13]

Trematoda

These parasites' name refers to the cavities in their

These are often called flukes, as most have flat

Adults range between 0.2 mm (0.0079 in) and 6 mm (0.24 in) in length. Individual adult digeneans are of a single sex, and in some species slender females live in enclosed grooves that run along the bodies of the males, partially emerging to lay eggs. In all species the adults have complex reproductive systems, capable of producing between 10,000 and 100,000 times as many eggs as a free-living flatworm. In addition, the intermediate stages that live in snails reproduce asexually.[13]

Adults of different species infest different parts of the definitive host - for example the

Members of this small group have either a single divided sucker or a row of suckers that cover the underside.

Cercomeromorpha

These parasites attach themselves to their hosts by means of disks that bear crescent-shaped hooks. They are divided into the Monogenea and Cestoda groupings.^[13]

Monogenea

Of about 1,100 species of

In the majority of species, known as eucestodes ("true tapeworms"), the neck produces a chain of segments called proglottids via a process known as strobilation. As a result, the most mature proglottids are furthest from the scolex. Adults of Taenia saginata, which infests humans, can form proglottid chains over 20 metres (66 ft) long, although 4 metres (13 ft) is more typical. Each proglottid has both male and female reproductive organs. If the host's gut contains two or more adults of the same cestode species they generally fertilize each other, however, proglottids of the same worm can fertilize each other and even themselves. When the eggs are fully developed, the proglottids separate and are excreted by the host. The eucestode life cycle is less complex than that of digeneans, but varies depending on the species. For example:

• Adults of
  hemocoel (an internal cavity which is the central part of the circulatory system) where it attaches itself using three small hooks. If the copepod is eaten by a fish, the larva metamorphoses into a small, unsegmented tapeworm, drills through to the gut and grows into an adult.^[13]
• Various species of
  Taenia infest the guts of humans, cats and dogs. The juveniles use herbivores – such as pigs, cattle and rabbits – as intermediate hosts. Excreted proglottids release eggs that stick to grass leaves and hatch after being swallowed by a herbivore. The larva then makes its way to the herbivore's muscle tissue, where it metamorphoses into an oval worm about 10 millimetres (0.39 in) long, with a scolex that is kept internally. When the definitive host eats infested raw or undercooked meat from an intermediate host, the worm's scolex pops out and attaches itself to the gut, when the adult tapeworm develops.^[13]

Members of the smaller group known as Cestodaria have no scolex, do not produce proglottids, and have body shapes similar to those of diageneans. Cestodarians parasitize fish and turtles.^[5]

Classification and evolutionary relationships

See also:

List of bilateral animal orders

The relationships of Platyhelminthes to other Bilateria are shown in the phylogenetic tree:^[20]

Bilateria	Acoelomorpha Deuterostomia Protostomia Ecdysozoa Spiralia Gnathifera Platytrochozoa Rouphozoa Gastrotricha Platyhelminthes 270 mya Lophotrochozoa Mollusca Annelida 550 mya 580 mya

The internal relationships of Platyhelminthes are shown below. The tree is not fully resolved.^[29][30][31]

Platyhelminthes

Mucorhabda Catenulidea Catenulida Rhabditophora Macrostomorpha Haplopharyngida Macrostomida Trepaxonemata Amplimatricata Prorhynchida Polycladida Gnosonesimora Gnosonesimida Euneoophora Rhabdocoela Kalyptorhynchia Dalytyphloplanida Proseriata Acentrosomata Adiaphanida Prolecithophora Fecampiida Tricladida (planarians) Bothrioneodermata Bothrioplanata Bothrioplanida (freshwater) tapeworms ) parasitic

The oldest confidently identified parasitic flatworm fossils are

synapomorphies - distinguishing features that all Platyhelminthes (but no other animals) exhibit. This makes it difficult to work out their relationships with other groups of animals, as well as the relationships between different groups that are described as members of the Platyhelminthes.^[34]

The "traditional" view before the 1990s was that Platyhelminthes formed the

molluscs, annelids and chordates. Since then, molecular phylogenetics, which aims to work out evolutionary "family trees" by comparing different organisms' biochemicals such as DNA, RNA and proteins, has radically changed scientists' view of evolutionary relationships between animals.^[20] Flatworms are now recognized as secondarily simplified bilaterians.^[35]

Detailed morphological analyses of anatomical features in the mid-1980s, as well as molecular phylogenetics analyses since 2000 using different sections of DNA, agree that Acoelomorpha, consisting of Acoela (traditionally regarded as very simple "turbellarians"^[13]) and Nemertodermatida (another small group previously classified as "turbellarians"^[19]) are the sister group to all other bilaterians.^[20][21] However, a 2007 study concluded that Acoela and Nemertodermatida were two distinct groups of bilaterians.^[22]

Xenoturbella, a bilaterian whose only well-defined organ is a statocyst, was originally classified as a "primitive turbellarian".^[23] Later studies suggested it may instead be a deuterostome,^[24][36] but more detailed molecular phylogenetics have led to its classification as sister-group to the Acoelomorpha.^[37]

The Platyhelminthes excluding Acoelomorpha contain two main groups - Catenulida and Rhabditophora - both of which are generally agreed to be monophyletic (each contains all and only the descendants of an ancestor that is a member of the same group).^[21][29] Early molecular phylogenetics analyses of the Catenulida and Rhabditophora left uncertainties about whether these could be combined in a single monophyletic group; a study in 2008 concluded that they could, therefore Platyhelminthes could be redefined as Catenulida plus Rhabditophora, excluding the Acoelomorpha.^[21]

Other molecular phylogenetics analyses agree the redefined Platyhelminthes are most closely related to

Gastrotricha, and both are part of a grouping known as Platyzoa. Platyzoa are generally agreed to be at least closely related to the Lophotrochozoa, a superphylum that includes molluscs and annelid worms. The majority view is that Platyzoa are part of Lophotrochozoa, but a significant minority of researchers regard Platyzoa as a sister group of Lophotrochozoa.^[20]

It has been agreed since 1985 that each of the wholly parasitic platyhelminth groups (

paraphyletic, since it does not include the Neodermata although these are descendants of a sub-group of "turbellarians".^[40]

Evolution

An outline of the origins of the parasitic lifestyle has been proposed;[41] epithelial feeding monopisthocotyleans on fish hosts are basal in the Neodermata and were the first shift to parasitism from free living ancestors. The next evolutionary step was a dietary change from epithelium to blood. The last common ancestor of Digenea + Cestoda was monogenean and most likely sanguinivorous.

In several members of the order Rhabdocoela an endosymbiotic relationship with microalgae has evolved. Some species in the same order has also evolved kleptoplasty.^[42]

The earliest known fossils confidently classified as tapeworms have been dated to 270 million years ago, after being found in

placoderm and acanthodian fossils from the Devonian of Latvia, at least some of which might represent parasitic monogeneans.^[43]

Interaction with humans

Parasitism

chronic illness that can damage internal organs. It can impair the growth and cognitive development of children, increasing the risk of bladder cancer in adults. The disease is caused by several flukes of the genus Schistosoma, which can bore through human skin; those most at risk use infected bodies of water for recreation or laundry.^[27]

In 2000, an estimated 45 million people were infected with the beef tapeworm

Diphyllobothrium latum occasionally causes vitamin B₁₂ deficiency and, in severe cases, megaloblastic anemia.^[44]

The threat to humans in developed countries is rising as a result of social trends: the increase in

seagulls which feed on manure and sludge; the increasing popularity of raw or lightly cooked foods; imports of meat, seafood and salad vegetables from high-risk areas; and, as an underlying cause, reduced awareness of parasites compared with other public health issues such as pollution. In less-developed countries, inadequate sanitation and the use of human feces (night soil) as fertilizer or to enrich fish farm ponds continues to spread parasitic platyhelminths, whilst poorly designed water-supply and irrigation projects have provided additional channels for their spread. People in these countries usually cannot afford the cost of fuel required to cook food thoroughly enough to kill parasites. Controlling parasites that infect humans and livestock has become more difficult, as many species have become resistant to drugs that used to be effective, mainly for killing juveniles in meat.^[44] While poorer countries still struggle with unintentional infection, cases have been reported of intentional infection in the US by dieters who are desperate for rapid weight-loss.^[48]

Pests

There is concern in northwest Europe (including the British Isles) regarding the possible proliferation of the New Zealand

Further reading

• Campbell, Neil A. (1996). Biology (Fourth ed.). New York: Benjamin/Cummings Publishing. p. 599.
  ISBN 0-8053-1957-3
  .
• Crawley, John L.; van de Graff, Kent M., eds. (2002). A Photographic Atlas for the Zoology Laboratory (Fourth ed.). Colorado: Morton Publishing Company.
  ISBN 0-89582-613-5
  .
• The Columbia Electronic Encyclopedia (6th ed.). Columbia University Press. 2004. Retrieved 8 February 2005.
• Evers, Christine A.; Starr, Lisa (2006). Biology: Concepts and Applications (6th ed.). United States: Thomson.
  ISBN 0-534-46224-3
  .
• Saló, E.; Pineda, D.; Marsal, M.; Gonzalez, J.; Gremigni, V.; Batistoni, R. (2002). "Genetic network of the eye in Platyhelminthes: Expression and functional analysis of some players during planarian regeneration". Gene. 287 (1–2): 67–74.
  PMID 11992724
  .

External links

Wikimedia Commons has media related to Platyhelminthes.

Wikispecies has information related to Platyhelminthes.

• "Marine flatworms of the world".