Turbellaria

(invertebrate zoology) A class of the phylum Platyhelminthes having bodies that are elongate and flat to oval or circular in cross section.

(Free-living flatworms)

Phylum: Platyhelminthes

Class: Turbellaria

Number of families: 102

Thumbnail description
Mostly free-living flatworms with a cellular epidermis that is usually ciliated; the mouth leads to a stomodeal pharynx and incomplete gut

Evolution and systematics

The phylogeny of the platyhelminth classes is not clear. Recent morphological and molecular studies have generated numerous hypotheses as to their relation to each other and to other phyla. Traditionally, the class Turbellaria was thought to be the basal ancestor of the parasitic classes (Trematoda, Cestoda, Monogenea) within the phylum. However, some researchers believe that the parasitic classes should be separated into a separate phylum (Neodermata) based on their unique tegument, the neodermis that may be adaptive to a parasitic existence. Both morphological and molecular studies also suggest that the Turbellaria are paraphyletic and that the orders Acoela and Nemertodermatida should be placed into a separate phylum. The Acoela have a primitive nerve net (no brain as in other flatworms), a simple pharynx when present, and a syncytial cellular gut without a cavity, entolecithal ova, and a lack of protonephridia. They may be the closest relatives to the acoeloid ancestor that gave rise to bilateral metazoa. It also has been postulated that the acoelomate condition (no body cavity) of the other platyhelminths may be secondarily derived from more advanced protostomes. The Acoela and Nemertodermatida are considered a separate, distinct taxon.

The ten recognized orders constitute the remaining members of the class Turbellaria. There are more than 4,500 described species within the Acoela and Turbellaria combined; however, many species have yet to be discovered and described. The characteristics of each order is the following:

• Order Catenulida has a simple pharynx and sac-like gut; the mesenchyme is poorly differentiated; ova are enotlecithal. They are elongate forms that occur in freshwater and marine habitats.
• Order Haplopharyngida are small worms with a simple pharynx, the proboscis is simple and ventral to the anterior tip of body (reminiscent of nemereans), anal pore is weakly developed; the brain is encapsulated with two ventral-lateral nerve cords; the ovary is simple without accessory organs; the male pore has a circle of hard straight stylets, anterior to female pore; the are free living and marine. They were once considered to be macrostomids. They contain two species.
• Order Lecithoepitheliata's ordinal status is questionable; the pharynx is complex and somewhat variable in the anterior of the body, the gut is simple; the ova are not entolecithal and surrounded by vitelline cells. There are about 30 species.
• Order Macrostomida have a simple pharynx, gut is a simple sac; the posterior end of the body may be broadened into an adhesive disc; there is no asexual zooid formation; the ovaries are often paired, the eggs entolecithal; the male pore is usually separate. They are mostly small interstitial marine and freshwater forms.

• Order Polycladida have ruffled plicate pharynx, the gut is multibranched with diverticula that may be anastomosing; the ovaries are scattered, with entolecithal ova; testes are scattered follicular, the male pore is usually anterior to the female pore. They are mostly large free-living marine flatworms that may be brightly colored.
• Order Prolecithophora's ordinal status is questionable. They have a plicate or bulbous pharynx, the gut is simple; the ovaries, and testes are follicular or compact, vitellaria is diffuse, eggs are ectolecithal. They are small, free-living or commensal, freshwater and marine forms.
• Order Proplicastomata is similar to the Acoela based on a few specimens; they have an elongate plicate pharynx; no statocysts; entolecithal ova. They are free-living marine forms.
• Order Proseriata is closely related to Tricladida. They have a cylindrical plicate pharynx, a simple gut, small, compact, paired ovaries at the end of the vitelline duct, the vitellaria is arranged along duct, ectolecithal ova. They are free-living marine forms.
• Order Rhabdocoela is a large diverse group with four suborders (Dalyellioida, Typhloplanoida, Kalptorhynchia, and Temnocephalida). They have a bulbous pharynx, a simple sac-like gut; the mesenchyme is fairly open; protonephridea are paired when present; the anterior brain and ventral nerve trunks are usually with cross connections, no statocyst; testes are compact; ovaries separate or joined with vitellaria, ova ectolecithal, a uterus is sometimes present. They have marine, freshwater, and terrestrial forms, many of which are symbiotic.
• Order Tricladida have a cylindrical plicate pharynx posteriorly directed, gut has one anterior and two posterior branches and numerous diverticula; mesenchyme is thick; no statocyst; male and female copulatory structures are complex, posterior to pharynx; follicular testes; one pair of small ovaries is usually anterior, vitellaria is extensive over most of lateral body, ectolecithal ova. They are usually large, flattened, and sometimes elongate worms with marine, freshwater, and terrestrial forms.

Physical characteristics

The class Turbellaria share the following characteristics with other classes within the Platyhelminthes:

• triblobastic (three tissue layers)
• acoelomate (no fluid filled body cavity or coelom)
• bilaterally symmetrical
• dorsoventrally flattened
• spiral cleavage and mesoderm derived from the 4d cell
• complex, incomplete gut (no anus)
• cephalized, with cerebral ganglion (brain) and longitudinal nerve cords that form a ladder-like nervous system
• numerous sense organs at the anterior end of the body, and tactile receptors distributed over the body, especially around the pharynx
• protonephridia that function in excretion and osmoregulation
• no circulatory system, which restricts the size and shape of these animals
• hermaphroditic with complex reproductive system

Turbellarians also are free-living or commensal (not usually parasitic), usually aquatic, and have a stomodeal pharynx. Their cellular epidermis is usually ciliated and contains mucous secreting cells and structures called rhabdoids that can produce copious mucus to prevent desiccation. Most turbellarians also have pigment-cup occelli for detecting light; some have an anterior pair where larger species may posses numerous pairs along the body.

Distribution

Turbellarian species are distributed worldwide, mostly in freshwater and marine environments with a few taxa occurring on land.

Habitat

Many of the minute species occur interstitially between grains of sand in aquatic habitats. Larger species are pelagic (marine) or live among submerged substrates such as rocks, coral, and algae. Many species, especially of the order Rhabodocoela, are symbiotic with various invertebrates and fishes. A few genera of the order Tricladida are terrestrial, living in damp leaf litter and soil.

Behavior

Turbellarians display a number of behaviors that prevent them from straying beyond their normal habitats and allows to them to maintain orientation within those habitats. For instance, most turbellarians are positively thigmotactic (touch) ventrally and negatively thigmotactic dorsally. This allows them to maintain their ventral side against the substrate in benthic forms. In other species where touch may not be the best way to orient to a substrate (such as interstitial and pelagic forms), they have statocysts so that they can orient to gravity (geotaxis). Most species are also negatively phototactic, which prevents worms from coming out in the daylight where they may get eaten or dry out in the case of terrestrial forms.

All turbellarians have a strong sense of smell that can be used to find food or mates (chemotaxis). Chemosensors are concentrated on each side of the head to help them determine the direction that the chemical trail is coming from. The heads of freshwater species are often expanded into auricles that have sensors. Some species have tentacles and ciliated pits to assist in chemotaxis. Dugesia swings its head back and forth to help determine the proper direction of the food source. Other species use trial and error to determine the proper direction to find food. They move in one direction until the signal gets weaker, and then continue switching direction until the signal is strongest. Some species also have been shown to orient to currents in order to find food (rheotaxis).

Feeding ecology and diet

Most turbellarians are carnivorous predators or scavengers. Carnivores feed on organisms that they can fit into their mouths, such as protozoans, copepods, small worms, and minute mollusks. Some species use mucus that may have poisonous or narcotic chemicals to slow or entangle prey. Some have specific diets and feed on sponges, ectoprocts, barnacles, and tunicates. Several species have commensal relationships with various invertebrates and few actually border on being parasitic because they graze on their live hosts. Terrestrial species feed on earthworms and land snails. A few species feed on microalgae that may be incorporated into the body, forming a symbiotic relationship in which the algae supply the worm with carbohydrates and fats and the worm supplies the algae with nitrogen waste products and a safe haven.

The pharynx and gut cells produce digestive enzymes that breakdown food extracellularly. Nutritive cells in the gastrodermis then phagotize partially digested material that is distributed throughout the body. Because these worm lack a circulatory system, larger species have extensive anastomosing guts to aid in distribution. Since these worms have incomplete guts, all waste must pass back out of the mouth.

Reproductive biology

Asexual reproduction is a common method of reproduction in freshwater and terrestrial turbellarians. Many of the triclads divide by transverse fission: the body splits transversely behind the pharynx and each part generates the rest of the body. The posterior portion attaches to the substrate and the anterior portion crawls away until it tears in two. In species such as Dugesia, the cells tend to vary in their ability to regenerate. The cells in the middle portion of the body have the strongest ability to regenerate. Experiments have shown that if just the tail is cut off, it will not grow a new body, whereas the main portion of the body will regenerate a new tail. The ability of the fissioned portions of these worms to regenerate the proper half has interested scientists for years in investigating why the head portion grows a tail and why the tail portion regenerates a new head. In the genera Catenula, Microstomum, and Stenostomum (orders Catenulida and Macrostomida), multiple transverse planes develop that lead to a train of individuals called zooids that do not detach until they reach a certain stage. Other species (e.g., Phagocata and some terrestrial species) detach fragments that become encysted and eventually develop into new individuals.

Turbellarians are hermaphroditic and their sexual reproductive systems are quite complicated. The male system may have one, two, or multiple testes that drain via sperm ducts that may lead to a storage area called the seminal vesicle. Prostate glands may be present that produce seminal fluid that mixes with the sperm in the seminal vesicle. The sperm then exits the worm via the protrusible penis or eversible cirrus with help from a muscular ejaculatory duct. The female system is more variable among the Turbellaria, depending on whether they produce entolecithal (Macrostomida and Polycladida) or ectolecithal (Rhabdocoela, Prolecithophora, and Tricladida) ova. A germovitellarium, which may be single or paired, produces entolecithal ova (yolk reserves within ova). A germarium or ovary, which is separate from the vitellaria, produces yolk-free ova that eventually are surrounded by separate yolk cells in a tanned protein capsule to form the ectolecithal egg. Sperm also are included within the egg capsule to insure fertilization. Eggs pass through the oviduct that may be differentiated into a seminal receptacle or uterus before deposition.

Cross fertilization (mating) usually occurs when worms align themselves with each other, and the cirrus or penis of each worm is inserted into the female gonopore or atrium of the other and deposits sperm. The worms then go their own way with the sperm stored in their seminal receptacles. In some species, mating occurs by hypodermic impregnation in which the male copulatory organ penetrates the body wall of the mate and deposits sperm in the mesenchyme. The sperm then make their way to the ova.

Turbellarians have either direct development or produce a pelagic larva. Polyclads often produce a pelagic Muller's larva that settles to the bottom and goes through metamorphosis in a few days. This larva has eight ventrally directed ciliated lobes, which it uses to swim. Stylochus, a parasitic polyclad, produces the Gotte's larva, which has only four ciliated lobes.

Conservation status

No turbellarians are considered threatened by the IUCN Red Book.

Significance to humans

The regenerative abilities of Dugesia have been studied extensively by scientists to better understand the healing and cell regeneration processes in humans. Several species parasitize commercially important species such as oysters and a few species cause pathological problems in marine ornamental fishes kept in aquaria.

Species accounts

Resources

Brusca R. C., and G. J. Brusca. Invertebrates. 2nd ed.

Sunderland, MA: Sinauer Associates, Inc., 2003.

Cannon, L. R. G. Turbellaria of the World—A Guide to Families and Genera. Queensland, Australia: Queensland Museum, 1986.

Kearn, G. C. Parasitism and the Platyhelminths. New York: Oxford University Press, 1998.

Prudhoe, S. A Monograph on Polyclad Turbellaria. New York: Pemberley books, 1985.

Sluys, R. A Monograph of the Marine Triclads. Brookfield, MA: A. A. Balkema, 1989.

[Article by: Dennis A. Thoney, PhD]

A class of the phylum Platyhelminthes commonly known as the flatworms. These animals are chiefly free-living and have simple life histories. The bodies are elongate and flat to oval or circular in cross section. Their length ranges from less than 0.04 in. (1 mm) to several inches, but may exceed 20 in. (50 cm) in land planaria. Large forms are often brightly colored. This class, which numbers some 3400 described species, is ordinarily subdivided into the orders Acoela, with 200 species; Rhabdocoela, 1110 species; Alloeocoela, 350 species; Tricladida, 1000 species; and Polycladida, 750 species. Although widely distributed in fresh and salt water and moist soil, they are usually overlooked because of their generally small size, secretive habits, and inconspicuous color. See also Acoela; Alloeocoela; Polycladida; Rhabdocoela; Tricladida.

Turbellaria
A marine species, Bedford's Flatworm, Pseudobiceros bedfordi
Scientific classification
Kingdom: Animalia Phylum: Platyhelminthes Class: Turbellaria Ehrenberg, 1831
Orders
Acoela Catenulida Haplopharyngida Lecithoepitheliata Macrostomida Nemertodermata Polycladida Prolecithophora Rhabdocoela Seriata Temnocephalida

The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from 1 mm (0.039 in) to 600 mm (24 in) long. All are flat with ribbon-like or leaf-like shapes, since their lack of respiratory and circulatory systems means that they have to rely on diffusion for internal transport. Most are predators, and all live in water or in moist terrestrial environments. All reproduce sexually and all are hermaphrodites.

Acoelomorpha and the genus Xenoturbella were included in the Turbellaria but are no longer regarded as Platyhelminthes. All the exclusively parasitic Platyhelminthes form a monophyletic group Neodermata, and it is agreed that these are descended from one small sub-group within the free-living Platyhelminthes. Hence the "Turbellaria" as traditionally defined are paraphyletic.

Contents 1 Description 1.1 Features common to all Platyhelminthes 1.2 Features specific to Turbelleria 2 Classification and evolutionary relationships 3 References

Description

Traditional invertebrate zoology divides the Platyhelminthes into four groups: Turbellaria and the wholly parasitic Trematoda, Monogenea and Cestoda.^[1] In this classification Turbellaria includes the Acoelomorpha (Acoela and Nemertodermatida). The name "Turbellaria" refers to the "whirlpools" of microscopic particles created close the skins of aquatic species by the movement of their cilia.^[2]

Features common to all Platyhelminthes

Main article: Platyhelminthes

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,^[1] while the radially symmetrical cnidarians and ctenophore have only two cell layers.^[3] Unlike other bilaterians, platyhelminthes have no internal body cavity and are therefore described as acoelomates. They also lack specialized circulatory and respiratory organs ^[1][2]

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.^[4] 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.^[1]

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 are used in regenerating tissues after injury or asexual reproduction.^[1]

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.^[2] 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).^[1]

All animals need to keep the concentration of dissolved substances in their body fluids at a fairly constant 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.^[1][5]

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.^[1][2]

Features specific to Turbelleria

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,^[2] 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.^[1]

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.^[2]

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.^[2]

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.^[1] 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.^[2]

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.^[2]

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. The planarian Dugesia is a well-known representative of class Turbellaria.^[2]

All turbellarians are hermaphrodites, in other words have both female and male reproductive cells, and fertilize eggs internally by copulation.^[2] 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.^[6] In most species "miniature adults" emerge when the eggs hatch, but a few large species produce plankton-like larvae.^[2]

Classification and evolutionary relationships

Main article: Platyhelminthes#Phylogeny

Platyhelminthes	Catenulida Rhabditophora various Rhabditophora various Rhabditophora various Rhabditophora Neodermata (all parasitic: flukes, tapeworms, etc.)

Relationships of Platyhelminthes (excluding Acoelomorpha) to each other^[7]

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"^[2]) and Nemertodermatida (another small group previously classified as "turbellarians"^[8]) are the sister group to all other bilaterians, including the rest of the "Platyhelminthes".^[9][10]

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.^[7][10] 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".^[10]

It has been agreed since 1985 that each of the wholly parasitic platyhelminth groups (Cestoda, 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.^[11] It is also generally agreed that the Neodermata are a relatively small sub-group a few levels down in the "family tree" of the Rhabditophora.^[10] 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".^[12]

References

1. ^ ^{a b c d e f g h i j} Hinde, R.T., (2001). "The Platyhelminthes". in Anderson, D.T.,. Invertebrate Zoology. Oxford University Press. pp. 58–80. ISBN 0195513681. 
2. ^ ^{a b c d e f g h i j k l m} Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 226–269. ISBN 0030259827. 
3. ^ Hinde, R.T. (2001). "The Cnidaria and Ctenophora". in Anderson, D.T.,. Invertebrate Zoology. Oxford University Press. pp. 28–57. ISBN 0195513681. 
4. ^ Barnes, R.S.K. (1998). The Diversity of Living Organisms. Blackwell Publishing. pp. 194-195. ISBN 0632049170. http://books.google.co.uk/books?id=Xvk-yrjXH70C&pg=PA194&dq=Platyhelminthes&lr=#PPA194,M1. Retrieved 2008-12-21. 
5. ^ Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 196–224. ISBN 0030259827. 
6. ^ Leslie Newman. "Fighting to mate: flatworm penis fencing". PBS. http://www.pbs.org/kcet/shapeoflife/episodes/hunt_explo2.html. Retrieved 2008-12-21. 
7. ^ ^{a b} Timothy, D., Littlewood, J., Telford, M.J., and Bray, R.A. (2004). "Protostomes and Platyhelminthes". in Cracraft, J., and Donoghue, M.J.. Assembling the Tree of Life. Oxford University Press US. pp. 209-223. ISBN 0195172345. http://books.google.co.uk/books?id=_jLl8zIRzucC&pg=PA217&lpg=PA217&dq=neodermata+phylogeny&source=web&ots=MvGJkNeshE&sig=cvNdFRhrV5erlT4lS96tY3on7IY&hl=en&sa=X&oi=book_result&resnum=10&ct=result#PPA223,M1. Retrieved 2008-12-23. 
8. ^ Jondelius, U., Ruiz-Trillo, I., Baguñà, J., Riutort, M. (April 2002). "The Nemertodermatida are basal bilaterians and not members of the Platyhelminthes". Zoologica Scripta 31 (2): 201-215. doi:10.1046/j.1463-6409.2002.00090.x. 
9. ^ Halanych, K.M. (December 2004). "The New View of Animal Phylogeny". Annual Review of Ecology, Evolution, and Systematics 35: 229-256. doi:10.1146/annurev.ecolsys.35.112202.130124. http://www.auburn.edu/academic/science_math/cosam/departments/biology/faculty/webpages/zzhalanych/Pub.pdfs/Halanych2004.pdf. Retrieved 2008-12-23. 
10. ^ ^{a b c d} Larsson, K., and Jondelius, U. (December 2008). "Phylogeny of Catenulida and support for Platyhelminthes". Organisms Diversity & Evolution 8: 378-387. doi:10.1016/j.ode.2008.09.002. 
11. ^ 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 2008-12-23. 
12. ^ Ehlers, U. (January 1986). "Comments on a phylogenetic system of the Platyhelminthes". Hydrobiologia 132 (1 pages=1-12). doi:10.1007/BF00046222. 

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