Platyhelminthes
Fossil range: No fossils known
Bedford's flatworm, Pseudobiceros bedfordi
Bedford's flatworm, Pseudobiceros bedfordi
Scientific classification
Kingdom: Animalia
Subkingdom: Eumetazoa
Superphylum: Platyzoa
Phylum: Platyhelminthes
Gegenbaur, 1859
Classes

Monogenea
Trematoda
Cestoda
Turbellaria

The flatworms (Phylum Platyhelminthes from the Greek πλατύ, platy, meaning "flat" and ἕλμινς (root: ἑλμινθ-), helminth-, meaning worm) are a phylum of relatively simple, unsegmented[1] soft-bodied invertebrate animals. There are probably over 100,000 extant species,[1] making them the largest phylum of acoelomates.[citation needed] They are thought to have a basal position among the lophotrochozoa[2], although their phylogenetic position is currently very uncertain and the "phylum" may be paraphyletic.[1] Over half of all known flatworm species are parasitic.[1] They occupy a range of habitats, including marine, freshwater, and even damp terrestrial environments.

Some flatworms have serious negative effects on human health and ecosystems. A troublesome terrestrial example is the New Zealand flatworm, Arthurdendyus triangulatus, which rapidly colonized large areas of Ireland and Scotland, eating most of the indigenous earthworms.[3] There are four classes: Trematoda (flukes, probably paraphyletic), Cestoda (tapeworms), Monogenea, and Turbellaria.

Contents

[edit] Description

[edit] Distinguishing features

Platyhelminthes are bilaterally symmetrical animals, in other words their left and right sides are mirror images of each other; this also implies that they have distinct top and bottom surfaces and distinct head and tail ends. Like other bilaterians they have three main cell layers,[4] while the radially symmetrical cnidarians and ctenophore have only two cell layers.[5] Beyond that, they are "defined more by what they do not have than by any particular series of bodily specializations."[6] Unlike other bilaterians, platyhelminthes have no internal body cavity and are therefore described as acoelomates. They also lack specialized circulatory and respiratory organs [4][7]

  Cnidarians[5] Ctenophores [5] Platyhelminthes [4][7] More "advanced" bilaterians[8]
Cnidocytes Yes No
Colloblasts No Yes No
Bilateral symmetry No Yes
Number of main cell layers Two, with jelly-like layer between them Three
Distinct brain No Yes
Specialized digestive system No Yes
Specialized excretory system No Yes
Body cavity containing internal organs No Yes
Specialized circulatory and respiratory organs No Yes

[edit] Features common to all sub-groups

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. The guts of large species have many branches, so that nutrients can diffuse to all parts of the body.[6] Respiration through the whole surface of the body makes platyhelminthes 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 parasites within other animals.[4]

The space between the skin and gut is filled with mesenchyme, a connective tissue that is 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 and are used in regenerating tissues after injury or asexual reproduction.[4]

Most platyhelminths have no anus and regurgitate undigested material through the mouth. 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.[7]The gut is lined with a single layer of endodermal cells which absorb and digest food. Some species break up and soften food first by secreting enzymes in the gut or the pharynx (throat).[4]

All animals need to keep the concentration of dissolved subtances in their body fluids at a fairly contant level. Internal parasites and free-living marine animals live in environments that have 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 control the level of concentration in their body fluids. Flame cells, so called because the beating of their flagella looks like a flickering candle flame, extract from the mesenchyme water that contains wastes and some re-usable 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 nephridiopores, while their microvilli re-absorb re-usable materials and as much water as is needed to keep the body fluids at the right level of concentration. These combinations of flame cells and tuble cells are called protonephredia.[4][8]

In all platyhelminths the nervous system is concentrated at the head end. This is least marked in the acoels, which have nerve nets rather like those of cnidarians and ctenophores, but densest around the head. Other platyhelminths have rings of ganglia in the head and main nerve trunks running along their bodies.[4][7]

[edit] Major sub-groups

Traditional inverterbrate zoology divides the platyhelminthes into four groups: Turbellaria, Trematoda, Monogenea and Cestoda. In this classification Turbellaria includes the Acoelomorpha.[4]

[edit] Turbellaria

Main article: Turbellaria
The turbellarian Pseudoceros dimidiatus
Two turbellarians mating by penis fencing. Each has two penises, the white spikes on the undersides of their heads.

These have about 4,500 species,[7] are mostly free-living, and range from 1 mm (0.039 in) to 600 mm (24 in) 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 symbiotes of other animals such as crustaceans, and some are parasites. Free-living turbellarians are mostly black, brown or gray, but some larger ones are brightly colored.[4]

Turbellarians have no cuticle (external layer of organic but non-cellular material). In a few species the skin is a syncitium, a collection of cells with multiple nuclei and a single shared external membrane. However the skins of most species consist of a single layer of cells, each of which generally has multiple cilia (small mobile "hairs"), although in some large species the upper surface has no cilia. These skins are also covered with microvilli between the cilia. They have many glands, usually submerged in the muscle layers below the skin and connect to the surface by pores through which they secrete mucus, adhesives and other substances. [7]

Small aquatic species use the cilia for locomotion, while larger ones use muscular movements of the whole body or of a specialized sole to creep or swim. Some are capable of burrowing, anchoring their rear ends at the bottom of the burrow and then stretching the head up to feed and then pulling it back down for safety. Some terrestrial species throw a thread of mucus which they use as a rope to climb from one leaf to another.[7]

The acoel Convoluta roscoffensis swallows cells of the green alga Tetraselmis and does not feed as an adult, presumably relying on the alge to provide nourishment as endosymbionts. In other acoels the gut is lined by a syncitium. These and some other 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 the underside. Most other turbellarians have a pharynx that is eversible, in other words can be extended by being turned inside-out, and the mouths of different species can be anywhere along the underside.[4] 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.[7]

Most turbellarians have pigment-cup ocelli ("little eyes"), one pair in most species but two or even three pairs in some. A few large species have many eyes in clusters over the brain, mounted on tentacles, or spaced uniformly round the edge of the body. The ocelli can only distinguish the direction from which light is coming and enable the animals to avoid it. A few groups – mainly catenulids, acoelomorphs and seriates – have statocysts, fluid-filled chambers containing a small solid particle or, in a few groups, two. These statocysts are thought to be balance and acceleration sensors, as that is the function they perform in cnidarian medusae and in ctenophores. However turbellarian statocysts have no sensory cilia, and it is unknown how they sense the movements and positions of the solid particles. 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 probably smell-sensors.[7]

Planaria 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 that were closest to the original head. This suggests that the growth of a head is controlled by a chemical whose concentration diminishes from head to tail. Many turbellarians clone themselves by tranverse or longitudinal division, and others, especially acoels, reproduce by budding.[7]

All turbellarians are hermaphrodites, in other words have both female and male reproductive cells, and fertilize eggs internally by copulation.[7] 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.[9] In most species "miniature adults" emerge when the eggs hatch, but a few large species produce plankton-like larvae.[7]

[edit] Trematoda

Main article: Trematoda
Life-cycle of the diagenean Metagonimus

These parasites' name refers to the cavity in their holdfasts (Greek τρῆμα, hole),[4] which resemble suckers.[10] The skin of all species is a syncitium. Trematodes are divided into two groups, Digenea and Aspidogastrea (also known as Aspodibothrea).[7]

[edit] Digenea

Main article: Digenea

These are often called flukes as most have flat rhomboid shapes like that of a flounder (Old English flóc). They have about 11,000 species, more than all other platyhelminthes combined, and second only to roundworms among parasites on metazoans.[7] Adults usually have two holdfasts, a ring round the mouth and a larger sucker midway along what would be the underside in a free-living flatworm.[4] Although the name "Digeneans" means "two generations", most have very complex lifecycles with up to seven stages, depending on what combinations of environments the early stages encounter – most importantly whether the eggs are deposited on land or in water. The definitive host in which adults develop is a land vertebrate, the earliest host of juvenile stages is usually a snail that may live on land or in water, and in many cases a fish or arthropod is the second host. The other stages transfer the parasites from one host to another.[7]

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, and partially emerge to lay eggs. In all species the adults have complex reproductive systems and can produce 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.[7]

Adults of different species infest different parts of the definitive host, for example the intestine, lungs, large blood vessels,[4] and liver.[7] The adults use a relatively large, muscular pharynx to ingest cells, cell fragments, mucus, body fluids or blood. In both the adults and the stages that life in snails, the external syncytium absorbs dissolved nutrients from the host. Adult digeneans can live without oxygen for long periods.[7]

[edit] Aspidogastrea

Main article: Aspidogastrea

Members of this small group have either a single divided sucker or a row of suckers that cover the underside.[7] They infest the guts of bony or cartilaginous fish and of turtles, and the body cavities of marine and freshwater bivalves and gastropods.[4] Their eggs produce ciliated swimming larvae, and the life-cycle has one or two hosts.[7]

[edit] Cercomeromorpha

This group of parasites attach themselves to the host by means of disks that bear crescent-shaped hooks. They are divided into Monogea and Cestoda.[7]

[edit] Monogea

Main article: Monogea

There are about 1,100 species of monogeans. Most are external parasites that require particular host species, mainly fish but in some smoe cases amphibians or aquatic reptiles. However some are internal parasites. Adult monogeans have large attachment organs at the rear, haptors (Greek ἅπτειν, haptein, means "catch"), which have suckers and hooks. To minimize water-resistance they have flattened bodies. In some species the pharynx secretes enzymes that digests the host's skin, allowing the parasite to feed on blood and cellular debris. Others graze externally on mucus and flakes of the host's skin. The name "Monogenea" is based on the fact that these parasites have only one non-larval generation.[7]

[edit] Cestoda

Main article: Cestoda
Lifecycle of the eucestode Taenia. Inset 5 shows the scolex, which has 4 suckers round the sides and, in Taenia solium, a disk with hooks on the end. Inset 6 shows the whole body, in which the scolex is the tiny round tip in the top left corner.

These are often called tapeworms because of their flat, slender but very long bodies – the name "cestode" is derived from the Latin word cestus, which means "tape". The adults of all 3,400 cestode species are internal parasites in the organs of vertebrates, including fish, cats, dogs and humans. The head is generally tiny compared to the size of the whole animal, and forms a scolex that attaches the parasite to the lining of the host's gut. The commonest type of scolex has four suckers round the sides and a disk equipped with hooks at the end. However some species have more complex arrangements, for example Myzophyllobothrium's scolex looks rather rather like a part-peeled banana, with four sucker-like flaps on the sides and a group of four small suckers on short stalks at the end.[7]

Cestodes have no mouths or guts, and the syncitial skin absorbs nutrients – mainly carbohydrates and amino acids – from the host, and also disguises it chemically to avoid attacks by the host's immune system.[7] Shortage of carbohydrates in the host's diet stunts the growth of the parasites and kills some. Their metabolisms generally use simple but inefficient chemical processes, and the parasites compensate by consuming large amounts of food relative to their size.[4]

In the majority of species, known as eucestodes ("true tapeworms"), the neck produces a chain of segments called proglottids by a process known as strobilation. Hence 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; but proglottids of the same worm can fertilize each other and even fertilize 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 Diphyllobothrium infest fish, and the juveniles use copepod crustaceans as intermediate hosts. Excreted proglottids release their eggs into the water, and the eggs hatch into ciliated swimming larvae. If a larva is swallowed by a copepod, it sheds the cilia and the skin becomes a syncitium and the larvae makes its way into the copepod's hemocoel (internal cavity that is the main part of the circulatory system) and attached istelf with 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 becomes an adult.[7]
  • Various species of Taenia infest the guts of humans, cats and dogs. The juveniles use herbivores – for example 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 makes its way to the herbivore's muscles and metamorphoses into an oval worm about 10 millimetres (0.39 in) long, with a scolex that is kept inside. When the definitive host eats infested and raw or undercooked meat from an intermediate host, the worm's scolex pops out and attaches itself to the gut, and the adult tapeworm develops.[7]

A smaller group, known as Cestodaria, have no scolex, do not produce proglottids, and have body shapes like those of diageneans. Cestodarians parasitize fish and turtles.[4]

[edit] Classification and evolutionary relationships

Relationships of Platyhelminthes to other Bilateria:[11]
Note: Bold indicates members of traditional "Platyhelminthes".
Platyhelminthes

Catenulida

Rhabditophora

various Rhabditophora



various Rhabditophora



various Rhabditophora


Neodermata
(all parasitic: flukes, tapeworms, etc.)





Relationships of Platyhelminthes (excluding Acoelomorpha) to each other[12]

The oldest known platyhelminth specimens are schistosome eggs discovered in ancient Egyptian mummies, and there are no convincing older platyhelminth fossils.[10] The Platyhelminthes have very few synapomorphies, distinguishing features that all Platyhelminthes and no other animals have. This makes it difficult to work out both their relationships with other groups of animals and the relationships between different groups that are described as members of the Platyhelminthes.[13]

The "traditional" view before the 1990s was that Platyhelminthes formed the sister group to all the other bilaterians, which include for example arthropods, 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.[11] Detailed morphological analyses of anatomical features in the mid-1980s and molecular phylogenetics analyses since 2000 using different sections of DNA agree that Acoelomorpha, consisting of Acoela (traditionally regarded as very simple "turbellarians"[7]) and Nemertodermatida (another small group previously classified as "turbellarians"[14]) are the sister group to all other bilaterians, including the rest of the "Platyhelminthes".[11][15] However a study in 2007 concluded that Acoela and Nemertodermatida were two distinct groups of bilaterians, although it agreed that both are more closely related to cnidarians (jellyfish, etc.) than other bilaterians are.[16]

Xenoturbella, a bilaterian with whose only well-defined organ is a statocyst, was originally classified as a "primitive turbellarian".[17] However it has recently been re-classified as a deuterostome.[18]

The "Platyhelminthes" excluding "Acoelomorpha" contain two main groups, Catenulida and Rhabditophora, and it is generally agreed that both are monophyletic, in other words each contains all and only the descendants of an ancestor which is a member of the same group.[15][12] Early molecular phylogenetics analyses of the Catenulida and Rhabditophora left uncertainties about whether these could be combined in a single monophyletic group, but a study in 2008 concluded that they could, and therefore that "Platyhelminthes" could be redefined as Catenulida plus Rhabditophora, excluding the "Acoelomorpha".[15]

Other molecular phylogenetics analyses agree that the redefined "Platyhelminthes" are most closely related to Gastrotricha and that both are part of a grouping known as Platyzoa. It is generally agreed that the Platyzoa are at least closely related to the Lophotrochozoa, a super-phylum 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.[11]

It has been agreed since 1985 that each of the wholly parasitic platyhelminth groups (Cestoda and Monogenea) and Trematoda) is monophyletic, and that together these form a larger monophyletic grouping, the Neodermata, in which the adults of all members have syncitial skins.[19] However there is debate about whether the Cestoda and Monogenea can be combined as a larger monophyletic group, the Cercomeromorpha.[20][19] It is generally agreed that the Neodermata are a relatively small sub-group a few levels down in the "family tree" of the Rhabditophora.[15] Hence the traditional sub-phylum "Turbellaria" is paraphyletic, since it does not include the Neodermata although these are descendants of a sub-group of "turbellarians".[21]

[edit] Interaction with humans

[edit] Parasitism

Magnetic resonance image of a patient with neurocysticercosis demonstrating multiple cysticerci within the brain

Cestodes (tapeworms) and digeneans (flukes) cause important diseases in humans and their livestock, and monogeneans can cause serious losses of stocks in fish farms.[22] Schistosomiasis, also known as bilharzia or snail fever, is the second most devastating parasitic disease in tropical countries, behind malaria. The Carter Center estimates that 200 million people in 74 countries are infected with the disease, and half the victims live in Africa. The condition has a low mortality rate, but often is a chronic illness that can damage internal organs. It can impair the growth and cognitive development of children, and increase the risk of bladder cancer in adults. The disease is caused by several flukes of the genus Schistosoma, which can bore through human skin. The people most at risk are those who use infected bodies of water for recreation or laundry.[23]

In 2000 an estimated 45 million people were infected with the beef tapeworm Taenia saginata and 3 million with the pork tapeworm Taenia solium.[22] Infection of the digestive system by adult tapeworms does not cause serious symptoms,[24][25] but penetration of T. solium larvae into the central nervous system is the major cause of acquired epilepsy worldwide.[26] In 2000 about 39 million people were infected with trematodes (flukes) that naturally parasitize fish and crustaceans but can pass to humans who eat raw or lighty-cooked sea food. Infection of humans by the broad fish tapeworm Diphyllobothrium latum, occasionally causes vitamin B12 deficiency and, in severe cases, megaloblastic anemia.[22]

The threat to humans in developed countries is rising as a result of social trends: the increase in organic farming, which uses manure and sewage sludge rather than artificial fertilizers, and spreads parasites both directly and via seagulls which feed on manure and sludge; the increasing popularity of raw or lighty-cooked foods; imports of meat, sea food 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 as fertilizer and to enrich fish farm ponds continue to spread parasitic platyhelminthes, and poorly-designed water-supply and irrigation projects have provided additional channels for the spread of parasites. People in these countries often 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.[22]

[edit] Pests

There is concern about the proliferation in North Europe, including the British Isles, of the New Zealand planarian Arthurdendyus triangulatus, which preys on earthworms.[27]

[edit] Benefits

In Hawaii the planarian Endeavouria septemlineata has been used to control the imported giant African snail Achatina fulica, which was displacing native snails, and Platydemus manokwari has been used for the same purpose in Philipines, Indonesia, Hawaii, New Guinea and Guam. Although A. fulica has declined sharply in Hawaii, there are doubts about how much E. septemlineata contributed to this. On the other hand P. manokwari is given credit for severely reducing and in places exterminating A. fulica – achieving much greater success than most biological pest control programs, which generally aim for a low, stable population of the pest species. The ability of planarians to take different kinds of prey and to resist starvation may account for its ability to decimate A. fulica. However these abilities have raised concerns that planarians may themselves become a serious threat to native snails.[28]

[edit] References

  1. ^ a b c d Klaus Rohde (2001), "Platyhelminthes (flat worms)", Encyclopaedia of Life Sciences, doi:10.1038/npg.els.0001585 
  2. ^ Saló, E (2002). "Genetic network of the eye in Platyhelminthes: expression and functional analysis of some players during planarian regeneration". Gene 287: 67. doi:10.1016/S0378-1119(01)00863-0. 
  3. ^ Boag, B, K A Evans, G W Yeates, P M Johns & R Nielson (1995). "Assessment of the global potential distribution of the predatory land planarian Artioposthia triangulata (Dendy) (Tricladida: Terricola) from ecoclimatic data". New Zealand Journal of Zoology 22: 311–318. http://www.rsnz.org/publish/nzjz/1995/76.pdf. 
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  5. ^ a b c Hinde, R.T. (2001). "The Cnidaria and Ctenophora". in Anderson, D.T.,. Invertebrate Zoology. Oxford University Press. pp. 28–57. ISBN 0195513681. 
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  17. ^ E. Westblad (1949). "Xenoturbella bocki n.g., n.sp., a peculiar, primitive turbellarian type". Arkiv för Zoologi 1: 3–29. 
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    • S. J. Bourlat, T. Juliusdottir, C. J. Lowe, R. Freeman, J. Aronowicz, M. Kirschner, E. S. Lander, M. Thorndyke, H. Nakano, A. B. Kohn, A. Heyland, L. L. Moroz, R. R. Copley, M. J. Telford (2006). "Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida". Nature 444: 85–88. doi:10.1038/nature05241. 
  19. ^ a b Willems, W.R., Wallberg, A., Jondelius, U., et al. (November 2005). "Filling a gap in the phylogeny of flatworms: relationships within the Rhabdocoela (Platyhelminthes), inferred from 18S ribosomal DNA sequences". Zoologica Scripta 35 (1): 1-17. doi:10.1111/j.1963-6409.2005.00216.x. https://doclib.uhasselt.be/dspace/bitstream/1942/1609/1/Filling%20the%20gap.pdf. Retrieved on 23 December 2008. 
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[edit] Further reading

  • Campbell, Neil A., Biology: Fourth Edition (Benjamin/Cummings Publishing, New York; 1996; page 599) ISBN 0-8053-1957-3
  • Crawley, John L., and Kent M. Van De Graff. (editors); A Photographic Atlas for the Zoology Laboratory: Fourth Edition) (Morton Publishing Company; Colorado; 2002) ISBN 0-89582-613-5
  • The Columbia Electronic Encyclopedia, 6th ed. (Columbia University Press; 2004) [Retrieved 8 February 2005][1]
  • Evers, Christine A., Lisa Starr. Biology:Concepts and Applications. 6th ed. United States: