(′tər·bə′lar·ē·ə)

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

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(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

Notoplana acticola
Pseudoceros ferrugineus
Oyster leech
Kronborgia amphipodicola
Paravortex scrobiculariae
Temnocephala chilensis
Bdelloura candida
Land planarian
Freshwater planarian

Resources

Books:

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.

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IN BRIEF: n. - Free-living flatworms.

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