Planarium
Encyclopedia
Planarian is the common name given to many non-parasitic flatworm
s of Turbellaria
class
. It is also the common name for a member of the genus Planaria within the family Planariidae
. Sometimes it also refers to the genus Dugesia
.
Planaria are common to many parts of the world, living in both saltwater and freshwater
ponds and rivers. Some species are terrestrial
and are found under logs, in or on the soil, and on plants in humid areas.
These animals move by beating cilia on the ventral dermis
, allowing them to glide along on a film of mucus
. Some move by undulations of the whole body by the contractions of muscles built into the body membrane.
Some planarians exhibit an extraordinary ability to regenerate
lost body parts. For example, a planarian split lengthwise or crosswise will regenerate into two separate individuals. Planarians' length ranges from 1 to 20 mm (0.0393700787401575 to 0.78740157480315 in), and some planarian species have two eye-spots (also known as ocelli
) 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
s (ectoderm
, mesoderm
, and endoderm
), and are acoelomate (i.e. they have a very solid body with no body cavity
). They have a single-opening digestive tract, in Tricladida
planarians this consists of one anterior branch and two posterior branches.
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. Other common species used are the blackish Planaria maculata and Girardia dorotocephala. Recently, however, the species Schmidtea mediterranea
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 orthologs in the human genome.
, pharynx
, and a structure called a gastrovascular cavity. The mouth is located in the center 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 with their muscular mouths. Food passes from the mouth through the pharynx into the intestines where it is digested, and its nutrients then diffuse
to the rest of the body.
Planaria receive oxygen and release carbon dioxide by diffusion. The excretory system is made of many tubes with many flame cell
s and excretory pores on them. Flame cells remove unwanted liquids from the body by passing them through ducts that lead to excretory pores where waste is released on the dorsal surface of the planarian.
At the head of the planarian there is a ganglion
under the eyespots. This bi-lobed mass of nerve tissue, the cerebral ganglia, is sometimes referred to as the planarian brain
and has been shown to exhibit spontaneous electrophysiological oscillations, similar to the electroencephalographic (EEG
) activity of other animals. From the ganglion there are two nerve
cords which extend 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, gray, or white and is about a half inch (1.3 cm) long. 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 cilia (hairlike projections). There are no circulatory or respiratory systems; oxygen entering and carbon dioxide leaving the planarian's body diffuses through the body wall.
because it involves the formation and fusion of gametes. In asexual reproduction, the planarian detaches its tail end and each half regrows the lost parts by regeneration
, allowing neoblasts (adult stem cells) to divide and differentiate. However, several problems can occur with this, so this does not happen often. Instead, in sexual reproduction, each planarian transports its excretion to the other planarian, giving and receiving sperm
. Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. Sexual reproduction is desirable because it enhances the survival of the species by increasing the level of genetic diversity
.
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 1962, 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
. McConnell's results are now attributed to observer bias
. No blinded experiment has ever reproduced his results. Subsequent explanations of maze-running enhancements 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.
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s of Turbellaria
Turbellaria
The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes , and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to in length...
class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
. It is also the common name for a member of the genus Planaria within the family Planariidae
Planariidae
Planariidae is a family of Tricladida that inhabits the freshwater.The type genus is Planaria Müller, 1776.-Genera:Twelve genera of Planariidae are known:*Albiplanaria*Atrioplanaria*Crenobia*Galeocephala*Hymanella...
. Sometimes it also refers to the genus Dugesia
Dugesia
Dugesia is a genus of dugesiid flatworms containing some common representatives of the class Turbellaria. These common flatworms are found in freshwater habitats. Their digestion tract consists of a central tubular pharynx. Each branch consists of ceca, which delivers the nutrients to the body...
.
Planaria are common to many parts of the world, living in both saltwater and freshwater
Fresh Water
Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...
ponds and rivers. Some species are terrestrial
Terrestrial animal
Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...
and are found under logs, in or on the soil, and on plants in humid areas.
These animals move by beating cilia on the ventral dermis
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...
, allowing them to glide along on a film of mucus
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
. Some move by undulations of the whole body by the contractions of muscles built into the body membrane.
Some planarians exhibit an extraordinary ability to regenerate
Regeneration (biology)
In biology, regeneration is the process of renewal, restoration, and growth that makes genomes, cells, organs, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans. At its most...
lost body parts. For example, a planarian split lengthwise or crosswise will regenerate into two separate individuals. Planarians' length ranges from 1 to 20 mm (0.0393700787401575 to 0.78740157480315 in), and some planarian species have two eye-spots (also known as ocelli
Ocellus
A simple eye refers to a type of eye design or optical arrangement that contains a single lens which detect light. A "simple eye" is so-called in distinction from a multi-lensed "compound eye", and is not necessarily at all simple in the usual sense of the word...
) 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
Germ layer
A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...
s (ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
, mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
, and endoderm
Endoderm
Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and mesoderm , with the endoderm as the intermost layer...
), and are acoelomate (i.e. they have a very solid body with no body cavity
Body cavity
By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism. However, the term usually refers to the space located between an animal’s outer covering and the outer lining of the gut cavity, where internal organs develop...
). They have a single-opening digestive tract, in Tricladida
Tricladida
Tricladida is an order of the Turbellaria, a group of free-living flatworms.Tricladida is characterized by triply branched intestine and anteriorly situated ovaries, next to the brain. Today it is split into three suborders, according to their phylogenetic relationships: Maricola, Cavernicola...
planarians this consists of one anterior branch and two posterior branches.
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. Other common species used are the blackish Planaria maculata and Girardia dorotocephala. Recently, however, the species Schmidtea mediterranea
Schmidtea mediterranea
Schmidtea mediterranea is a freshwater planarian that lives in southern Europe and Tunisia. It is a model for regeneration and development of tissues such as the brain and germline....
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 orthologs in the human genome.
Anatomy and physiology
The planarian has very simple organ systems. The digestive system consists of a mouthMouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
, pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
, and a structure called a gastrovascular cavity. The mouth is located in the center 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 with their muscular mouths. Food passes from the mouth through the pharynx into the intestines where it is digested, and its nutrients then diffuse
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
to the rest of the body.
Planaria receive oxygen and release carbon dioxide by diffusion. The excretory system is made of many tubes with many flame cell
Flame cell
right|frameA 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, removing waste materials...
s and excretory pores on them. Flame cells remove unwanted liquids from the body by passing them through ducts that lead to excretory pores where waste is released on the dorsal surface of the planarian.
At the head of the planarian there is a ganglion
Ganglion
In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
under the eyespots. This bi-lobed mass of nerve tissue, the cerebral ganglia, is sometimes referred to as the planarian brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
and has been shown to exhibit spontaneous electrophysiological oscillations, similar to the electroencephalographic (EEG
EEG
EEG commonly refers to electroencephalography, a measurement of the electrical activity of the brain.EEG may also refer to:* Emperor Entertainment Group, a Hong Kong-based entertainment company...
) activity of other animals. From the ganglion there are two nerve
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
cords which extend 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, gray, or white and is about a half inch (1.3 cm) long. 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 cilia (hairlike projections). There are no circulatory or respiratory systems; oxygen entering and carbon dioxide leaving the planarian's body diffuses through the body wall.
Reproduction
Planaria are hermaphrodites, possessing both testicles and ovaries. Thus, one of their gametes will combine with the gamete of another planarian. This type of gamete fusion is sexual reproductionSexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
because it involves the formation and fusion of gametes. In asexual reproduction, the planarian detaches its tail end and each half regrows the lost parts by regeneration
Regeneration (biology)
In biology, regeneration is the process of renewal, restoration, and growth that makes genomes, cells, organs, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans. At its most...
, allowing neoblasts (adult stem cells) to divide and differentiate. However, several problems can occur with this, so this does not happen often. Instead, in sexual reproduction, each planarian transports its excretion to the other planarian, giving and receiving sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
. Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. Sexual reproduction is desirable because it enhances the survival of the species by increasing the level of genetic diversity
Genetic diversity
Genetic diversity, the level of biodiversity, refers to the total number of genetic characteristics in the genetic makeup of a species. It is distinguished from genetic variability, which describes the tendency of genetic characteristics to vary....
.
Biochemical memory experiments
In 1955, Robert Thompson and James V. McConnellJames V. McConnell
James V. McConnell 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....
conditioned
Classical conditioning
Classical conditioning is a form of conditioning that was first demonstrated by Ivan Pavlov...
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 1962, 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 RNA
Memory RNA is a now-discredited hypothetical form of RNA that was proposed by James V. McConnell and others as a means of explaining how long-term memories were stored in the brain...
. McConnell's results are now attributed to observer bias
Observer-expectancy effect
The observer-expectancy effect is a form of reactivity, in which a researcher's cognitive bias causes them to unconsciously influence the participants of an experiment...
. No blinded experiment has ever reproduced his results. Subsequent explanations of maze-running enhancements 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.