A planarian is one of the many
flatworm
The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegment ...
s of the traditional
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differentl ...
Turbellaria
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 to large freshwater forms more ...
.
It usually describes free-living flatworms of the order Tricladida (triclads), although this common name is also used for a wide number of free-living
platyhelminthes
The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a Phylum (biology), phylum of relati ...
.
Planaria are common to many parts of the world, living in both saltwater and
freshwater
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...
ponds and rivers. Some species are
terrestrial
Terrestrial refers to things related to land or the planet Earth.
Terrestrial may also refer to:
* Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on or near the ground, as opposed to ...
and are found under logs, in or on the soil, and on plants in humid areas.
The triclads are characterized by triply branched intestine and anteriorly situated ovaries, next to the brain. Today the order Tricladida is split into three suborders, according to their phylogenetic relationships:
Maricola
Maricola is a suborder of triclad flatworms including species that mainly inhabit salt water environments. However, some species are also known from freshwater or brackish waters.Ball, I. R. 1974. La Faune Terrestre de l'Ile de Saint Hélène: ...
,
Cavernicola and
Continenticola
Continenticola is a clade that includes the land planarians (Geoplanidae) and the freshwater triclads (Dendrocoelidae, Dugesiidae, Kenkiidae and Planariidae).
Phylogeny
On the basis of molecular evidences, Carranza and colleagues suggested in 19 ...
. Formerly, the Tricladida was split according to habitats:
Maricola
Maricola is a suborder of triclad flatworms including species that mainly inhabit salt water environments. However, some species are also known from freshwater or brackish waters.Ball, I. R. 1974. La Faune Terrestre de l'Ile de Saint Hélène: ...
, which is marine;
Paludicola which inhabits freshwater; and
Terricola
Geoplanidae is a family of flatworms known commonly as land planarians or land flatworms.
These flatworms are mainly predators of other invertebrates, which they hunt, attack and capture using physical force and the adhesive and digestive proper ...
, which is land-dwelling.
Planaria exhibit an extraordinary ability to
regenerate lost body parts. For example, a planarian split lengthwise or crosswise will regenerate into two separate individuals. Some planarian species have two eye-spots (also known as
ocelli
A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...
) that can detect the intensity of light, while others have several eye-spots. The eye-spots act as
photoreceptors and are used to move away from light sources. Planaria have three
germ layer
A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans (animals that are sister taxa to the sponges) produce two or three pr ...
s (
ectoderm
The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...
,
mesoderm
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...
, and
endoderm
Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...
), and are acoelomate (they have a very solid body with no
body cavity
A body cavity is any space or compartment, or potential space, in an animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid.
The two largest human body cavities are the ventral body cavity, and ...
). They have a single-opening digestive tract; in Tricladida planarians this consists of one anterior branch and two posterior branches.
Planarians move by beating
cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
on the
ventral
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...
dermis
The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...
, allowing them to glide along on a film of
mucus
Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...
. Some also may move by undulations of the whole body by the contractions of muscles built into the body membrane.
Triclads play an important role in watercourse ecosystems and are often very important as bio-indicators.
The most frequently used planarian in high school and first-year college laboratories is the brownish ''
Girardia tigrina
''Girardia tigrina'' is a species of dugesiid native to the Americas.Ball, I. R.: A contribution to the phylogeny and biogeography of the freshwater triclads (Platyhelminthes: Turbellaria). Biology of the Turbellaria (Edited by: Riser NW and Mor ...
''. Other common species used are the blackish ''
Planaria maculata
''Planaria'' is a genus of planarians in the family Planariidae. When an individual is cut into pieces, each piece has the ability to regenerate into a fully formed individual.
Description
Currently the genus ''Planaria'' is defined as fresh ...
'' and ''
Girardia dorotocephala
''Girardia dorotocephala'' is a species of dugesiid triclad native to North America. It has been accidentally introduced in Japan.Sluys, R., Kawakatsu, M., Yamamoto, K. (2010). "Exotic freshwater planarians currently known from Japan". ''Belgian ...
''. Recently, however, the species ''
Schmidtea mediterranea
''Schmidtea mediterranea'' is a freshwater triclad that lives in southern Europe and Tunisia. It is a model for regeneration, stem cells and development of tissues such as the brain and germline.
Distribution
''Schmidtea mediterranea'' is found ...
'' has emerged as the species of choice for modern molecular biological and genomic research due to its diploid chromosomes and the existence of both asexual and sexual strains. Recent genetic screens utilizing double-stranded RNA technology have uncovered 240 genes that affect regeneration in ''S. mediterranea''. Many of these genes have
ortholog
Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a spec ...
s in the human genome.
Anatomy and physiology
The planarian has very simple organ systems. The digestive system consists of a
mouth
In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on ...
,
pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...
, and a
gastrovascular cavity
The gastrovascular cavity is the primary organ of digestion and circulation in two major animal phyla: the Coelenterates or cnidarians (including jellyfish and corals) and Platyhelminthes (flatworms). The cavity may be extensively branched into ...
. The mouth is located in the middle of the underside of the body. Digestive enzymes are secreted from the mouth to begin external digestion. The pharynx connects the mouth to the gastrovascular cavity. This structure branches throughout the body allowing nutrients from food to reach all extremities.
Planaria eat living or dead small animals that they suck up with their muscular mouths. Food passes from the mouth through the pharynx into the intestines where it is digested by the cells lining the intestines. Then its nutrients
diffuse
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
to the rest of the planaria.
Planaria receive oxygen and release carbon dioxide by diffusion. The excretory system is made of many tubes with many
flame cell
A flame cell is a specialized excretory cell found in the simplest freshwater invertebrates, including flatworms, rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Flame cells function like a kidney, re ...
s and excretory pores on them. Also, flame cells remove unwanted liquids from the body by passing them through ducts which lead to excretory pores, where waste is released on the dorsal surface of the planarian.
The triclads have an anterior end or head where sense organs, such as eyes and
chemoreceptor
A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecept ...
s, are usually found. Some species have auricles that protrude from the margins of the head. The auricles can contain chemical and mechanical sensory receptors.
[Kenk, R., 1972. Freshwater planarians (Turbellarians) of North America.]
The number of eyes in the triclads is variable depending on the species. While many species have two eyes (e.g. ''
Dugesia
''Dugesia'' (pronounced, /duˈd͡ʒiʒ(i)ə/) is a genus of dugesiid triclads that contains some common representatives of the class Turbellaria. These common flatworms are found in freshwater habitats of Africa, Eurasia, and Australia. ''Dugesi ...
'' or ''
Microplana''), others have many more distributed along the body (e.g. most
Geoplaninae
Geoplaninae is a subfamily of land planarians endemic to the Neotropical region. However, one species, ''Obama nungara'' has been introduced in Europe.
Description
The subfamily Geoplaninae was initially defined by Ogren and Kawakatsu (1990)Ogre ...
). Sometimes, those species with two eyes may present smaller accessory or supernumerary eyes. The subterranean triclads are often eyeless or blind.
The body of the triclads is covered by a ciliated epidermis that contains
rhabdite
Rhabdites (from Greek, ''rhabdos'', rod) are rodlike structures in the cells of the epidermis or underlying parenchyma in certain turbellarians, and in the epidermis of nemertea
Nemertea is a phylum of animals also known as ribbon worms or p ...
s. Between the epidermis and the gastrodermis there is a parenchymatous tissue or
mesenchyme
Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every o ...
.
Nervous system
The head of the planarian evolved through
cephalisation
Cephalization is an evolutionary trend in which, over many generations, the mouth, sense organs, and nerve ganglia become concentrated at the front end of an animal, producing a head region. This is associated with movement and bilateral symmetr ...
; the planarian head has a
ganglion
A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympatheti ...
under the eyespots. The cerebral ganglia, a bi-lobed mass of nerve tissue, is sometimes referred to as the planarian “
brain
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
” and has been shown to exhibit spontaneous electrophysiological oscillations, similar to the electroencephalographic (
EEG
Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...
) activity of other animals. From the ganglion there are two
nerve
A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system.
A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...
cords which extend along the length of the tail. There are many transverse nerves connected to the nerve cords extending from the brain, which makes the nerve system look like a ladder. With a ladder-like nerve system, it is able to respond in a coordinated manner.
The planarian has a soft, flat, wedge-shaped body that may be black, brown, blue, gray, or white. The blunt, triangular head has two ocelli (eyespots), pigmented areas that are sensitive to light. There are two auricles (earlike projections) at the base of the head, which are sensitive to touch and the presence of certain chemicals. The mouth is located in the middle of the underside of the body, which is covered with hairlike projections (cilia). There are no circulatory or respiratory systems; oxygen enters and carbon dioxide leaves the planarian's body by diffusing through the body wall.
Reproduction
There are sexual and asexual planarian. Sexual planarian are
hermaphrodites
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many taxonomic groups of animals (mostly invertebrates) do not have separ ...
, possessing both testicles and ovaries. Thus, one of their gametes will combine with the gamete of another planarian. Each planarian transports its secretion to the other planarian, giving and receiving
sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
. Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. In asexual reproduction, the planarian detaches its tail end and each half regrows the lost parts by regeneration, allowing endoblasts (adult stem cells) to divide and differentiate, thus resulting in two worms. Some researchers claim that the products derived from bisecting planarian are similar to the products of planarian asexual reproduction; however, debates about the nature of asexual reproduction in planaria and its effect on the population are ongoing. Some species of planarian are exclusively asexual, whereas some can reproduce both sexually and asexually. In most of the cases the sexual reproduction involve two individuals; auto fecundation has been rarely reported (e.g. in ''
Cura foremanii
''Cura foremanii'' is a species of freshwater planarian belonging to the family Dugesiidae.Tyler, S., Artois, T.; Schilling, S.; Hooge, M.; Bush, L.F. (eds) (2006-2023). World List of turbellarian worms: Acoelomorpha, Catenulida, Rhabditophora'' ...
'').
As a model system in biological and biomedical research
The
life history of planarians make them a model system for investigating a number of biological processes, many of which may have implications for human health and disease. Advances in
molecular
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
genetic technologies has made the study of
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
function possible in these animals and scientists are studying them worldwide. Like other invertebrate model organisms, for example ''
C. elegans
''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (r ...
'' and ''
D. melanogaster
''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly" or "pomace fly". Starting with Ch ...
'', the relative simplicity of planarians facilitates experimental study.
Planarians have a number of cell types, tissues and simple organs that are
homologous to our own
cells
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
,
tissues and
organs
In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a fu ...
. However,
regeneration
Regeneration may refer to:
Science and technology
* Regeneration (biology), the ability to recreate lost or damaged cells, tissues, organs and limbs
* Regeneration (ecology), the ability of ecosystems to regenerate biomass, using photosynthesis
...
has attracted the most attention.
Thomas Hunt Morgan
Thomas Hunt Morgan (September 25, 1866 – December 4, 1945) was an American evolutionary biologist, geneticist, embryologist, and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating the role tha ...
was responsible for some of the first systematic studies (that still underpin modern research) before the advent of
molecular biology
Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including biomolecular synthesis, modification, mechanisms, and interactions. The study of chemical and physi ...
as a discipline.
Planarians are also an emerging model organism for
aging research. These animals have an apparently limitless regenerative capacity, and the asexual animals seem to maintain their
telomerase
Telomerase, also called terminal transferase, is a ribonucleoprotein that adds a species-dependent telomere repeat sequence to the 3' end of telomeres. A telomere is a region of repetitive sequences at each end of the chromosomes of most euka ...
levels throughout their lifetime, making them "effectively immortal".
Regeneration
Planarian can be cut into pieces, and each piece can regenerate into a complete organism. Cells at the location of the wound site proliferate to form a
blastema
A blastema (Greek ''βλάστημα'', "offspring") is a mass of cells capable of growth and regeneration into organs or body parts. The changing definition of the word "blastema" has been reviewed by Holland (2021). A broad survey of how blast ...
that will differentiate into new tissues and regenerate the missing parts of the piece of the cut planaria. It's this feature that gave them the famous designation of being "immortal under the edge of a knife." Very small pieces of the planarian, estimated to be as little as 1/279th of the organism it is cut from, can regenerate back into a complete organism over the course of a few weeks.
New tissues can grow due to
pluripotent Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta.
According to Latin terms, Pluripotentia means the ability for many thin ...
stem cell
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
s that have the ability to create all the various cell types. These adult stem cells are called
endoblasts, and comprise 20% or more of the cells in the adult animal.
They are the only proliferating cells in the worm, and they differentiate into progeny that replace older cells. In addition, existing
tissue is remodeled to restore symmetry and proportion of the new planaria that forms from a piece of a cut up organism.
[
The organism itself does not have to be completely cut into separate pieces for the regeneration phenomenon to be witnessed. In fact, if the head of a planarian is cut in half down its center, and each side retained on the organism, it is possible for the planarian to regenerate two heads and continue to live. Researchers, including those from ]Tufts University
Tufts University is a private research university on the border of Medford and Somerville, Massachusetts. It was founded in 1852 as Tufts College by Christian universalists who sought to provide a nonsectarian institution of higher learning. ...
in the U.S., sought to determine how microgravity and micro-geomagnetic fields would affect the growth and regeneration of planarian flatworm
The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegment ...
s, ''Dugesia japonica
''Dugesia japonica'' is a species of freshwater planarian that inhabits freshwater bodies of East Asia, including Japan, Korea, Taiwan, China and northeastern Siberia. However, molecular studies suggest that ''Dugesia japonica'' is polyphyletic ...
''. They discovered that one of the amputated fragments sent to space regenerated into a double-headed worm. The majority of such amputated worms (95%) did not do so, however. An amputated worm regenerated into a double-head creature after spending five weeks aboard the International Space Station (ISS) – though regeneration of amputated worms as double-headed heteromorphosis is not a rare phenomenon unique to a microgravity environment. In contrast, two-headed planaria regenerates can be induced by exposing amputated fragments to electrical fields. Such exposure with opposite polarity can induce a planarian with 2 tails. Two-headed planaria regenerates can be induced by treating amputated fragments with pharmacological agents that alter levels of calcium, cyclic AMP, and protein kinase C activity in cells, as well as by genetic expression blocks (interference RNA) to the canonical Wnt/β-Catenin signalling pathway.
Biochemical memory experiments
In 1955, Robert Thompson and James V. McConnell
James V. McConnell (October 26, 1925 – April 9, 1990) was an American biologist and animal psychologist. He is most known for his research on learning and memory transfer in planarians conducted in the 1950s and 1960s. McConnell also publish ...
conditioned planarian flatworms by pairing a bright light with an electric shock. After repeating this several times they took away the electric shock, and only exposed them to the bright light. The flatworms would react to the bright light as if they had been shocked. Thompson and McConnell found that if they cut the worm in two, and allowed both worms to regenerate each half would develop the light-shock reaction. In 1963, McConnell repeated the experiment, but instead of cutting the trained flatworms in two he ground them into small pieces and fed them to other flatworms. He reported that the flatworms learned to associate the bright light with a shock much faster than flatworms who had not been fed trained worms.
This experiment intended to show that memory could be transferred chemically. The experiment was repeated with mice, fish, and rats, but it always failed to produce the same results. The perceived explanation was that rather than memory being transferred to the other animals, it was the hormones in the ingested ground animals that changed the behavior. McConnell believed that this was evidence of a chemical basis for memory, which he identified as memory RNA
Memory transfer was a biological process proposed by James V. McConnell and others in the 1960s. Memory transfer proposes a chemical basis for memory termed memory RNA which can be passed down through flesh instead of an intact nervous system. Si ...
. McConnell's results are now attributed to observer bias Observer bias is one of the types of detection bias and is defined as any kind of systematic divergence from accurate facts during observation and the recording of data and information in studies. The definition can be further expanded upon to incl ...
.[For a general review, see also ] No blinded experiment
In a blind or blinded experiment, information which may influence the participants of the experiment is withheld until after the experiment is complete. Good blinding can reduce or eliminate experimental biases that arise from a participants' expec ...
has ever reproduced his results of planarians scrunching when exposed to light. Subsequent explanations of this scrunching behaviour associated with cannibalism of trained planarian worms were that the untrained flatworms were only following tracks left on the dirty glassware rather than absorbing the memory of their fodder.
In 2012, Tal Shomrat and Michael Levin have shown that planarians exhibit evidence of long-term memory retrieval after regenerating a new head.
Phylogeny and taxonomy
Phylogeny
Phylogenetic supertree after Sluys et al., 2009:
Taxonomy
Linnaean ranks after Sluys ''et al.'', 2009:
* Order Tricladida
** Suborder Maricola
Maricola is a suborder of triclad flatworms including species that mainly inhabit salt water environments. However, some species are also known from freshwater or brackish waters.Ball, I. R. 1974. La Faune Terrestre de l'Ile de Saint Hélène: ...
*** Superfamily Cercyroidea
Maricola is a suborder of triclad flatworms including species that mainly inhabit salt water environments. However, some species are also known from freshwater or brackish waters.Ball, I. R. 1974. La Faune Terrestre de l'Ile de Saint Hélène: T ...
**** Family Centrovarioplanidae
''Centrovarioplana tenuis'' is a species of Maricola triclad that is found in Antarctica. It is the only species known in the monotypic genus ''Centrovarioplana'' and the family
Family (from la, familia) is a group of people related either ...
**** Family Cercyridae
Cercyridae is a family (biology), family of Maricola triclads.
Taxonomy
List of known genera:
*''Stummeria''
*''Oregoniplana''
**Tribe Cercyrini
***''Probursa''
***''Pacifides''
***''Puiteca''
***''Cerbussowia''
***''Sabussowia''
***''Cercyra' ...
**** Family Meixnerididae
Meixnerididae is a family of Maricola triclad
A planarian is one of the many flatworms of the traditional class Turbellaria. It usually describes free-living flatworms of the order Tricladida (triclads), although this common name is also u ...
*** Superfamily Bdellouroidea
**** Family Uteriporidae
**** Family Bdellouridae
*** Superfamily Procerodoidea
**** Family Procerodidae
''Procerodes'' is a genus of marine triclads. It is the only genus in the monotypic family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship ...
** Suborder Cavernicola
*** Family Dimarcusidae
Dimarcusidae is a family of triclads found mostly in freshwater habitats of caves, although at least one species, '' Rhodax evelinae'', occurs in surface waters. Currently the family contains only seven species distributed in five genera, althou ...
** Suborder Continenticola
Continenticola is a clade that includes the land planarians (Geoplanidae) and the freshwater triclads (Dendrocoelidae, Dugesiidae, Kenkiidae and Planariidae).
Phylogeny
On the basis of molecular evidences, Carranza and colleagues suggested in 19 ...
*** Superfamily Planarioidea
Planarioidea is a superfamily of freshwater triclads that comprises the families Dendrocoelidae, Kenkiidae and Planariidae
Planariidae is a family (biology), family of freshwater Tricladida, planarians.
The type genus is ''Planaria'' Mülle ...
**** Family Planariidae
Planariidae is a family (biology), family of freshwater Tricladida, planarians.
The type genus is ''Planaria'' Müller, 1776.Ball IR: A contribution to the phylogeny and biogeography of the freshwater triclads (Platyhelminthes: Turbellaria). Bi ...
**** Family Dendrocoelidae
Dendrocoelidae is a family of freshwater tricladida flatworms that has a holarctic distribution.
The largest freshwater triclad known belongs to this family, it's up to 40 cm in length and inhabits the Lake Baikal.
Description
The famil ...
**** Family Kenkiidae
Kenkiidae is a family of freshwater triclads. Their species can be found sporadically in caves, groundwater, and deep lakes in Central Asia, Far East and North America.Ball, I. R.: A contribution to the phylogeny and biogeography of the freshwa ...
*** Superfamily Geoplanoidea
Geoplanoidea is a superfamily of freshwater and land triclads that comprises the species of the Geoplanidae and the Dugesiidae families.
Dugesiidae and Geoplanidae share a duplication of the cluster that codifies for the 18S ribosomal RNA
18S ...
**** Family Dugesiidae
Dugesiidae is a family of freshwater planarians distributed worldwide (except Antarctica). The type genus is ''Dugesia'' Girard, 1850.Ball, I. R.: A contribution to the phylogeny and biogeography of the freshwater triclads (Platyhelminthes: Tur ...
**** Family Geoplanidae
Geoplanidae is a family of flatworms known commonly as land planarians or land flatworms.
These flatworms are mainly predators of other invertebrates, which they hunt, attack and capture using physical force and the adhesive and digestive proper ...
See also
*
*
References
External links
More information on freshwater planarians and their biology
* YouTube videos
Planaria eating worm segment
Planarian
''Schmidtea mediterranea'', facts, anatomy, image
at GeoChemBio.com
Alejandro Sanchez-Alvarado's Seminar: Regeneration in Planarians
Link to an article discussing some work on planarian immortality
A user-friendly visualization tool and database of planarian regeneration experiments
*
Tricladida on the Encyclopedia of Life (EOL)
on the UF / IFAS Featured Creatures Web site
{{Authority control
Turbellaria
Animal models
Negligibly senescent organisms
Articles containing video clips
Invertebrate common names