Reading: Flatworms - Biology

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This laboratory exercise covers the following animals. You should learn this classification scheme and be able to classify the animals into these categories.

  • Phylum: Platyhelminthes (Flatworms)
    • Class: Turbellaria (planarians)
    • Class: Trematoda (Flukes)
    • Class: Cestoda (Tapeworms)


Flatworms are flattened and have bilateral symmetry.

They are triploblastic (have 3 embryonic tissue layers: ectoderm, mesoderm, and endoderm) and therefore have organ-level of organization. There is no body cavity, so they are acoelomate.

Flatworms have a gastrovascular cavity with one opening (a sac-like gut).

Free-living Species

Example: Dugesia—a freshwater planarian

Planarians have a branching sac-like gut (one opening).

The main function of the excretory system is for water regulation. It consists of two structures called protonephridia. Each protonephridium contains flame cells that move excess water into tubes that open to the outside.

Planarians have a head region with sense organs. The nervous system of Dugesia is somewhat more complex than the nerve net of Cnidarians. It consists of a brain and nerve cords arranged in a ladder-like configuration.

Planarians have ocelli (eyespots) allow the presence and intensity of light to be determined. These structures are covered but have an opening to one side and forward. They can tell the direction of light because shadows fall on some of the receptor cells while others are illuminated. They move away from light.

Planarians are hermaphroditic, that is, they contain both male and female sex organs. They can reproduce asexually simply by pinching in half; each half grows a new half.

Movement is accomplished by the use of cilia and also by muscular contractions.


Members of this group are primarily parasites (feed on a host species).

Parasitic forms lack cephalization.

The reproductive cycle typically involves two host species, a primary host and a secondary (or intermediate) host. Adults live in the primary host and larvae develop in the secondary host. The life cycle often alternates between sexual and asexual reproduction.

Liver flukes are found in vertebrate livers.

Nearly half of people in the tropics have blood flukes. Schistosomiasis is a blood fluke that afflicts 200 million people in the world. The secondary host is a snail.


  1. Place a living planarian on a watch glass and observe its movements under a dissecting microscope. Look for the eyespots, auricles, gastrovascular cavity, and pharynx.
  2. Planarians cannot see images but they can tell the direction of light with their eyespots. Cover 1/2 of the watch glass with aluminum foil. Does the planarian favor the light area or the dark area?
  3. View a slide of a preserved planarian and note the eyespots, auricles, gastrovascular cavity, and pharynx.

Figure 1. Left: Planarian anterior end X 40. Middle: Planarian digestive tract mid section X 40. Right: Planarian c.s. X 40.

Liver Flukes

Observe either a preserved liver fluke or a slide of a liver fluke using a dissecting microscope.

Figure 2. Left: Sheep liver fluke (Fasciola hepatica) stained. Right: Liver fluke (preserved)


  1. View a preserved tapeworm (Taenia).
  2. View slides of Taenia. Locate the scolex. View a gravid (filled with eggs) proglottid.

Tapeworms live in the intestines of vertebrates.

They may reach 10 m in length (>30 feet). They have no digestive or nervous tissue. Attachment to the intestinal wall is by a scolex, a structure that contains hooks and suckers.

Figure 3. Taenia scolex X 40

Figure 4. Taenia (preserved)

The segments (proglottids) each contain male and female reproductive organs. Eggs are fertilized from sperm, which often come from other proglottids of the same individual. After fertilization, other organs within the proglottid disintegrate and the proglottid becomes filled with eggs.

The intermediate hosts are usually pigs or cattle. They can become infected by drinking water contaminated with human feces.

Tapeworms can be passed to humans in undercooked meat, especially pork.

The photographs below show the scolex and proglottids at increasing distances from the scolex. Those segments closest to the scolex (on the left) are the smallest. Those furthest away (on the right) become filled with zygotes, break away, and pass out with the feces.

Figure 5. Left: Taenia pisiformis anterior end. Middle: Taenia pisiformis mid region. Right: Taenia pisiformis posterior end



  • Flatworms, Biology 102. Authored by: Michael J. Gregory, Ph.D..


    In one way, the flatworm s “are better” than a sea slug, Yusa says.

    Planarians are cross-eyed flatworm s that biology students used to cut into pieces to study regeneration.

    In one sense, planarians, the little cross-eyed flatworm s that biology students mince up to study regeneration, “are better,” Yusa says.

    An initiative in Senegal, for example, will reintroduce edible native river prawns that prey on the snails that transmit the parasitic flatworm that causes schistosomiasis.

    In the 1950s, an unknown psychology professor at the University of Michigan named James McConnell made headlines—and eventually became something of a celebrity—with a series of experiments on freshwater flatworm s called planaria.

    Such is seen in the life history of the liver fluke, a flatworm which kills sheep, and in the tapeworm.

    A certain fresh-water flatworm has the mouth and pharynx in the middle of the body.

    The parasite that's doing the damage is a flatworm , a trematode called Hepatodirus hominis.

    If a flatworm be cut in two, the front piece grows out a new tail, the hind piece a new head, and two perfect worms result.

    Notes on Phylum Platyhelminthes | Zoology

    The term Platyhelminthes “flatworms” was first proposed by Gaugenbaur (1859) and applied to the animals now included under that heading. The name Platyhelminthes has been derived from the Greek platys = flat + helmins = worms. At first, nemertines and others were also included but later were removed to other groups.

    The phylum is now restricted to three classes, Turbellaria, Trematoda, and Cestoda. Although these classes have many structural differences, they all show enough similarity in the body pattern to indicate a common origin.

    Platyhelminthes include the flatworms, their bodies are compressed dorsoventrally and show bilateral symmetry. They are the lowest triploblastic acoelomate Metazoa, but they are more advanced than Coelenterata because their tissues are organised into organs.

    The mesoderm forms a type of connective tissue called parenchyma which fills the body spaces between the ectoderm and endoderm so that there is no coelom or haemocoele, hence, they are called acoelomate animals, mesoderm also forms organs, such as the excretory and reproductive organs.

    The excretory system has one or two canals with branches, the finer branches end in flame cells, the canal has no internal opening but it opens to the exterior only. Blood vascular system and respiratory system are absent. There is no anus and in some even the mouth and alimentary canal are absent.

    The nervous system consists of a network, but it has ganglia at the anterior end which serves as a brain. Reproductive organs are very highly developed, most Platyhelminthes are hermaphrodite.

    The phylum includes some 15,000 species, and it is divided into three classes. Class Turbellaria includes ciliated flatworms which are generally free-living, Trematoda are non-ciliated parasitic flatworms or flukes, while Cestoda are all endoparasitic flatworms or tapeworms.

    The typical structure of Platyhelminthes is seen only in Turbellaria, because the Trematoda and Cestoda, due to parasitic habit, have become different from their free-living ancestors, they have lost their ciliated epidermis and have acquired a cuticle and organs of attachment.

    The trematodes have retained the body form and alimentary canal of Turbellaria, but the tapeworms have become elongated into a chain and the alimentary canal is lost.

    18 Questions and Answers on Flatworms

    The most well-known representatives of platyhelminthes are worms that cause human diseases, such as taenia and schistosome. Planaria, since it has been extensively studied in Biology, is also well-known.

    Platyhelminth  Morphology

    More Bite-Sized Q&As Below

    2. What is the main external morphological feature that differentiates platyhelminthes from other worms (nematodes)?

    Platyhelminthes are also known as flatworms because they are worms with a flat body. This is the main external morphological feature that differentiates them from nematodes (roundworms).

    3. How many germ layers make up the body of platyhelminthes? How are they classified according to this feature?

    Platyhelminthes are the first triploblastic animals (remember that cnidarians are diploblastic), meaning that they contain three germ layers: the ectoderm, mesoderm and endoderm.

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    Platyhelminth Physiology

    4. What are the types of digestion and digestive system of platyhelminthes?

    Flatworms have incomplete digestive systems and use extracellular and intracellular complementary digestion.

    5. How are nutrients distributed by the digestive system in planaria?

    Planaria have a single-opening digestive system (incomplete) with branches that transport nutrients to all parts of the body.

    6. How is gas exchange carried out in flatworms?

    Platyhelminthes exchange gases exclusively by diffusion through their body surface. This is only possible because all cells are located relatively near their exterior, since gases diffuse cell by cell (the flat shape of these worms is a feature that allows this type of respiration).

    7. Poriferans and cnidarians do not have excretory systems. Do platyhelminthes have an excretory system?

    Platyhelminthes have a primitive excretory system made of flame cells (also called solenocytes), excretory ducts and excretory pores.

    8. What is an example of a freshwater flatworm? What physiological problem must these animals solve when living in that environment?

    Freshwater platyhelminthes, such as planaria, have an internal environment much more concentrated in solutes than their exterior environment and, as a result, have a tendency to gain water. These organisms then need a drainage system to avoid cell death caused by excessive water.

    This problem is solved by the presence of protonephridia located along longitudinal channels in their body. Protonephridia have ciliated cells, which are called flame cells, that push water outside the body through excretory pores.

    9. Is the nervous system of platyhelminthes more or less sophisticated than that of cnidarians? What are the main neural structures found in flatworms? How is this neural organization important for the diversity of ecological niches explored by species of the phylum?

    Platyhelminthes have a more sophisticated nervous system than cnidarians, as they contain neural chords with ganglia (grouping of neurons) appear, a characteristic of the evolutionary process of increased nervous complexity. In platyhelminthes, the beginning of the cephalization process can be seen, with a concentration of neurons (nervous cells) in the anterior portion of the body and the appearance of photoreceptor cells in the ocelli.

    Due to the increased capacity of these animals to perceive and to interact with their environment, thanks to the increased complexity of their neural network, it is possible to find platyhelminthes in a variety of environments and ways of life, including the terrestrial, and with diverse ways of life, including parasitic and free-living species.

    10. What is cephalization? How does lateral symmetry favor cephalization?

    Cephalization is the evolutionary tendency for concentration of nervous control in central structures in which neurons are grouped (including the brain and ganglia formations). Evolutionarily, the cephalization process begins with the appearance of ganglia (groups of neurons) in platyhelminthes and reaches peak in vertebrates, animals with a skull to protect a well-developed brain.

    Through lateral symmetry, the body can be divided into lateral portions: superior, inferior, anterior and posterior. These portions must be integrated and controlled in some manner and this need caused the appearance of ganglial complexity and of organisms with a head, a privileged extremity of the bilateral body where the nervous central command and important sensory organs are located.

    Reproduction in Flatworms

    11. Do platyhelminthes use sexual or asexual reproduction?

    Platyhelminthes may present sexual or asexual reproduction.

    12. How can asexual reproduction in planaria be described?

    Planaria can divide asexually by transverse bipartition due to the great regeneration capability of their tissues. When they attach to a substrate, they can induce a constriction in their middle region, separating their body into two parts, each of which produces a new specimen as tissue regenerates. 

    13. Are flatworms monoecious or dioecious?

    There are monoecious hermaphrodite flatworms, such as planaria and taenias, as well as dioecious (having male and female specimens) species, such as schistosomes.

    14. Is it possible for a hermaphrodite species to carry out cross-fertilization?

    Hermaphrodite species of animals and plants carry out cross-fertilization mainly due to the maturation of female and male structures at different periods.

    Cross-fertilization occurs in planaria, which are hermaphrodites in which sexual fertilization takes place with male and female gametes from different specimens. These specimens bring their sexaul structures together and exchange gametes.

    15. What is direct development?਍o planaria have a larval stage?

    Sexual reproduction with direct development is the type of sexual reproduction in which there is no larval stage of embryonic development. When a larval stage exists, it is called indirect development.

    There is no larval stage in the sexual reproduction of planaria.

    Platyhelminth Classes

    16. Into which classes are platyhelminthes divided? How can these classes be described and what are some representative species in each of them?

    Platyhelminthes are divided into three classes: turbellarians (or Turbellaria), trematodes (or Trematoda) and cestodes (or Cestoda).

    Turbellarians are free-living platyhelminthes and their main representative is planaria (Dugesia tigrina). Trematodes are parasites, which live inside a host,. The schistosome (Schistosoma mansoni) that causes schistosomiasis is an example of one. Cestodes are also parasites. They have no digestive tract and their cells are nourished through the absorption of nutrients from their host. Their most well-known representative species are beef and pork taenia (Taenia saginata and Taenia solium), which are human parasites.

    17. What are the main human diseases caused by platyhelminthes?

    The main human diseases caused by platyhelminthes are schistosomiasis, tapeworm disease (cestodiasis) and cysticercosis.

    (Note:Diseases are studied in the “Diseases” section of this site.)

    Flatworms Summary

    18. The main features of platyhelminthes. How can platyhelminthes be described according to examples of representative species, basic morphology, type of symmetry, germ layers and coelom, digestive system, respiratory system, circulatory system, excretory system, nervous system and types of reproduction?

    Examples of representative species: planaria, schistosomes, taenia. Basic morphology: flat worm. Type of symmetry: bilateral. Germ layers and coelom: triploblastic, acoelomates. Digestive system: incomplete. Respiratory system: nonexistent, respiration by diffusion. Circulatory system: nonexistent. Excretory system: protonephridia with flame cells. Nervous system: ganglial, beginning of cephalization. Types of reproduction: asexual and sexual.


    Distinguishing features Edit

    Platyhelminthes are bilaterally symmetrical animals: their left and right sides are mirror images of each other this also implies they have distinct top and bottom surfaces and distinct head and tail ends. Like other bilaterians, they have three main cell layers (endoderm, mesoderm, and ectoderm), [4] while the radially symmetrical cnidarians and ctenophores (comb jellies) have only two cell layers. [5] Beyond that, they are "defined more by what they do not have than by any particular series of specializations." [6] Unlike other bilaterians, Platyhelminthes have no internal body cavity, so are described as acoelomates. They also lack specialized circulatory and respiratory organs, both of these facts are defining features when classifying a flatworm's anatomy. [4] [7] Their bodies are soft and unsegmented. [8]

    Attribute Cnidarians and Ctenophores [5] Platyhelminthes (flatworms) [4] [7] More "advanced" bilaterians [9]
    Bilateral symmetry No Yes
    Number of main cell layers Two, with jelly-like layer between them Three
    Distinct brain No Yes
    Specialized digestive system No Yes
    Specialized excretory system No Yes
    Body cavity containing internal organs No Yes
    Specialized circulatory and respiratory organs No Yes

    Features common to all subgroups Edit

    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, allowing nutrients to diffuse to all parts of the body. [6] Respiration through the whole surface of the body makes them vulnerable to fluid loss, and restricts them to environments where dehydration is unlikely: sea and freshwater, moist terrestrial environments such as leaf litter or between grains of soil, and as parasites within other animals. [4]

    The space between the skin and gut is filled with mesenchyme, also known as parenchyma, a connective tissue 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. [4]

    Most platyhelminths have no anus and regurgitate undigested material through the mouth. However, some long species have an anus and some with complex, branched guts have more than one anus, since excretion only through the mouth would be difficult for them. [7] The gut is lined with a single layer of endodermal cells that absorb and digest food. Some species break up and soften food first by secreting enzymes in the gut or pharynx (throat). [4]

    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 with 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 concentration of 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 reusable 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 reabsorb reusable materials and as much water as is needed to keep the body fluids at the right concentration. These combinations of flame cells and tube cells are called protonephridia. [4] [9]

    In all platyhelminths, the nervous system is concentrated at the head end. Other platyhelminths have rings of ganglia in the head and main nerve trunks running along their bodies. [4] [7]

    Early classification divided the flatworms in four groups: Turbellaria, Trematoda, Monogenea and Cestoda. This classification had long been recognized to be artificial, and in 1985, Ehlers [10] proposed a phylogenetically more correct classification, where the massively polyphyletic "Turbellaria" was split into a dozen orders, and Trematoda, Monogenea and Cestoda were joined in the new order Neodermata. However, the classification presented here is the early, traditional, classification, as it still is the one used everywhere except in scientific articles. [4] [11]

    Turbellaria Edit

    These have about 4,500 species, [7] are mostly free-living, and range from 1 mm (0.04 in) to 600 mm (24 in) in length. Most are predators or scavengers, and terrestrial species are mostly nocturnal and live in shaded, humid locations, such as leaf litter or rotting wood. However, some are symbiotes of other animals, such as crustaceans, and some are parasites. Free-living turbellarians are mostly black, brown or gray, but some larger ones are brightly colored. [4] The Acoela and Nemertodermatida were traditionally regarded as turbellarians, [7] [12] but are now regarded as members of a separate phylum, the Acoelomorpha, [13] [14] or as two separate phyla. [15] Xenoturbella, a genus of very simple animals, [16] has also been reclassified as a separate phylum. [17]

    Some 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 their undersides. Most other turbellarians have a pharynx that is eversible (can be extended by being turned inside-out), and the mouths of different species can be anywhere along the underside. [4] The freshwater species Microstomum caudatum can open its mouth almost as wide as its body is long, to swallow prey about as large as itself. [7]

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

    Planarians, a subgroup of seriates, 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 fragments which were originally located closest to the original head. This suggests the growth of a head is controlled by a chemical whose concentration diminishes throughout the organism, from head to tail. Many turbellarians clone themselves by transverse or longitudinal division, whilst others, reproduce by budding. [7]

    The vast majority of turbellarians are hermaphrodites (they have both female and male reproductive cells) which fertilize eggs internally by copulation. [7] Some of the larger aquatic species mate by penis fencing – a duel in which each tries to impregnate the other, and the loser adopts the female role of developing the eggs. [18] In most species, "miniature adults" emerge when the eggs hatch, but a few large species produce plankton-like larvae. [7]

    Trematoda Edit

    These parasites' name refers to the cavities in their holdfasts (Greek τρῆμα, hole), [4] which resemble suckers and anchor them within their hosts. [8] The skin of all species is a syncitium, which is a layer of cells that shares a single external membrane. Trematodes are divided into two groups, Digenea and Aspidogastrea (also known as Aspodibothrea). [7]

    Digenea Edit

    These are often called flukes, as most have flat rhomboid shapes like that of a flounder (Old English flóc). There are about 11,000 species, more than all other platyhelminthes combined, and second only to roundworms among parasites on metazoans. [7] Adults usually have two holdfasts: a ring around the mouth and a larger sucker midway along what would be the underside in a free-living flatworm. [4] Although the name "Digeneans" means "two generations", most have very complex life cycles with up to seven stages, depending on what combinations of environments the early stages encounter – the most important factor being whether the eggs are deposited on land or in water. The intermediate stages transfer the parasites from one host to another. The definitive host in which adults develop is a land vertebrate the earliest host of juvenile stages is usually a snail that may live on land or in water, whilst in many cases, a fish or arthropod is the second host. [7] For example, the adjoining illustration shows the life cycle of the intestinal fluke metagonimus, which hatches in the intestine of a snail, then moves to a fish where it penetrates the body and encysts in the flesh, then migrating to the small intestine of a land animal that eats the fish raw, finally generating eggs that are excreted and ingested by snails, thereby completing the cycle. A similar life cycle occurs with Opisthorchis viverrini, which is found in South East Asia and can infect the liver of humans, causing Cholangiocarcinoma (bile duct cancer). Schistosomes, which cause the devastating tropical disease bilharzia, also belong to this group. [19]

    Adults range between 0.2 mm (0.0079 in) and 6 mm (0.24 in) in length. Individual adult digeneans are of a single sex, and in some species slender females live in enclosed grooves that run along the bodies of the males, partially emerging to lay eggs. In all species the adults have complex reproductive systems, capable of producing between 10,000 and 100,000 times as many eggs as a free-living flatworm. In addition, the intermediate stages that live in snails reproduce asexually. [7]

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

    Aspidogastrea Edit

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

    Cercomeromorpha Edit

    These parasites attach themselves to their hosts by means of disks that bear crescent-shaped hooks. They are divided into the Monogenea and Cestoda groupings. [7]

    Monogenea Edit

    Of about 1,100 species of monogeneans, most are external parasites that require particular host species - mainly fish, but in some cases amphibians or aquatic reptiles. However, a few are internal parasites. Adult monogeneans have large attachment organs at the rear, known as haptors (Greek ἅπτειν, haptein, means "catch"), which have suckers, clamps, and hooks. They often have flattened bodies. In some species, the pharynx secretes enzymes to digest the host's skin, allowing the parasite to feed on blood and cellular debris. Others graze externally on mucus and flakes of the hosts' skins. The name "Monogenea" is based on the fact that these parasites have only one nonlarval generation. [7]

    Cestoda Edit

    These are often called tapeworms because of their flat, slender but very long bodies – the name "cestode" is derived from the Latin word cestus, which means "tape". The adults of all 3,400 cestode species are internal parasites. Cestodes have no mouths or guts, and the syncitial skin absorbs nutrients – mainly carbohydrates and amino acids – from the host, and also disguises it chemically to avoid attacks by the host's immune system. [7] Shortage of carbohydrates in the host's diet stunts the growth of parasites and may even kill them. Their metabolisms generally use simple but inefficient chemical processes, compensating for this inefficiency by consuming large amounts of food relative to their physical size. [4]

    In the majority of species, known as eucestodes ("true tapeworms"), the neck produces a chain of segments called proglottids via a process known as strobilation. As a result, the most mature proglottids are furthest from the scolex. Adults of Taenia saginata, which infests humans, can form proglottid chains over 20 metres (66 ft) long, although 4 metres (13 ft) is more typical. Each proglottid has both male and female reproductive organs. If the host's gut contains two or more adults of the same cestode species they generally fertilize each other, however, proglottids of the same worm can fertilize each other and even themselves. When the eggs are fully developed, the proglottids separate and are excreted by the host. The eucestode life cycle is less complex than that of digeneans, but varies depending on the species. For example:

    • Adults of Diphyllobothrium infest fish, and the juveniles use copepod crustaceans as intermediate hosts. Excreted proglottids release their eggs into the water where the eggs hatch into ciliated, swimming larvae. If a larva is swallowed by a copepod, it sheds the cilia and the skin becomes a syncitium the larva then makes its way into the copepod's hemocoel (an internal cavity which is the central part of the circulatory system) where it attaches itself using three small hooks. If the copepod is eaten by a fish, the larva metamorphoses into a small, unsegmented tapeworm, drills through to the gut and grows into an adult. [7]
    • Various species of Taenia infest the guts of humans, cats and dogs. The juveniles use herbivores – such as pigs, cattle and rabbits – as intermediate hosts. Excreted proglottids release eggs that stick to grass leaves and hatch after being swallowed by a herbivore. The larva then makes its way to the herbivore's muscle tissue, where it metamorphoses into an oval worm about 10 millimetres (0.39 in) long, with a scolex that is kept internally. When the definitive host eats infested raw or undercooked meat from an intermediate host, the worm's scolex pops out and attaches itself to the gut, when the adult tapeworm develops. [7]

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

    The relationships of Platyhelminthes to other Bilateria are shown in the phylogenetic tree: [13]

    The internal relationships of Platyhelminthes are shown below. The tree is not fully resolved. [21] [22] [23]

    The oldest confidently identified parasitic flatworm fossils are cestode eggs found in a Permian shark coprolite, but helminth hooks still attached to Devonian acanthodians and placoderms might also represent parasitic flatworms with simple life cycles. [24] The oldest known free-living platyhelminth specimen is a fossil preserved in Eocene age Baltic amber and placed in the monotypic species Micropalaeosoma balticus, [25] whilst the oldest subfossil specimens are schistosome eggs discovered in ancient Egyptian mummies. [8] The Platyhelminthes have very few synapomorphies - distinguishing features that all Platyhelminthes (but no other animals) exhibit. This makes it difficult to work out their relationships with other groups of animals, as well as the relationships between different groups that are described as members of the Platyhelminthes. [26]

    The "traditional" view before the 1990s was that Platyhelminthes formed the sister group to all the other bilaterians, which include, for instance, arthropods, molluscs, annelids and chordates. Since then, molecular phylogenetics, which aims to work out evolutionary "family trees" by comparing different organisms' biochemicals such as DNA, RNA and proteins, has radically changed scientists' view of evolutionary relationships between animals. [13] Detailed morphological analyses of anatomical features in the mid-1980s, as well as molecular phylogenetics analyses since 2000 using different sections of DNA, agree that Acoelomorpha, consisting of Acoela (traditionally regarded as very simple "turbellarians" [7] ) and Nemertodermatida (another small group previously classified as "turbellarians" [12] ) are the sister group to all other bilaterians, including the rest of the Platyhelminthes. [13] [14] However, a 2007 study concluded that Acoela and Nemertodermatida were two distinct groups of bilaterians, although it agreed that both are more closely related to cnidarians (jellyfish, etc.) than other bilaterians are. [15]

    Xenoturbella, a bilaterian whose only well-defined organ is a statocyst, was originally classified as a "primitive turbellarian". [16] Later studies suggested it may instead be a deuterostome, [17] [27] but more detailed molecular phylogenetics have led to its classification as sister-group to the Acoelomorpha. [28]

    The Platyhelminthes excluding Acoelomorpha contain two main groups - Catenulida and Rhabditophora - both of which are generally agreed to be monophyletic (each contains all and only the descendants of an ancestor that is a member of the same group). [14] [21] Early molecular phylogenetics analyses of the Catenulida and Rhabditophora left uncertainties about whether these could be combined in a single monophyletic group a study in 2008 concluded that they could, therefore Platyhelminthes could be redefined as Catenulida plus Rhabditophora, excluding the Acoelomorpha. [14]

    Other molecular phylogenetics analyses agree the redefined Platyhelminthes are most closely related to Gastrotricha, and both are part of a grouping known as Platyzoa. Platyzoa are generally agreed to be at least closely related to the Lophotrochozoa, a superphylum that includes molluscs and annelid worms. The majority view is that Platyzoa are part of Lophotrochozoa, but a significant minority of researchers regard Platyzoa as a sister group of Lophotrochozoa. [13]

    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 syncytial skins. [29] However, there is debate about whether the Cestoda and Monogenea can be combined as an intermediate monophyletic group, the Cercomeromorpha, within the Neodermata. [29] [30] It is generally agreed that the Neodermata are a sub-group a few levels down in the "family tree" of the Rhabditophora. [14] 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". [31]

    An outline of the origins of the parasitic life style has been proposed [32] epithelial feeding monopisthocotyleans on fish hosts are basal in the Neodermata and were the first shift to parasitism from free living ancestors. The next evolutionary step was a dietary change from epithelium to blood. The last common ancestor of Digenea + Cestoda was monogenean and most likely sanguinivorous.

    The earliest known fossils confidently classified as tapeworms have been dated to 270 million years ago , after being found in coprolites (fossilised faeces) from an elasmobranch. [1] Putative older fossils include brownish bodies on the bedding planes reported from the Late Ordovician (Katian) Vauréal Formation (Canada) by Knaust & Desrochers (2019), tentatively interpreted as turbellarians (though the authors cautioned that they might ultimately turn out to be fossils of acoelomorphs or nemerteans) [2] and circlets of fossil hooks preserved with placoderm and acanthodian fossils from the Devonian of Latvia, at least some of which might represent parasitic monogeneans. [33]

    Parasitism Edit

    Cestodes (tapeworms) and digeneans (flukes) cause diseases in humans and their livestock, whilst monogeneans can cause serious losses of stocks in fish farms. [34] Schistosomiasis, also known as bilharzia or snail fever, is the second-most devastating parasitic disease in tropical countries, behind malaria. The Carter Center estimated 200 million people in 74 countries are infected with the disease, and half the victims live in Africa. The condition has a low mortality rate, but usually presents as a chronic illness that can damage internal organs. It can impair the growth and cognitive development of children, increasing the risk of bladder cancer in adults. The disease is caused by several flukes of the genus Schistosoma, which can bore through human skin those most at risk use infected bodies of water for recreation or laundry. [19]

    In 2000, an estimated 45 million people were infected with the beef tapeworm Taenia saginata and 3 million with the pork tapeworm Taenia solium. [34] Infection of the digestive system by adult tapeworms causes abdominal symptoms that, whilst unpleasant, are seldom disabling or life-threatening. [35] [36] However, neurocysticercosis resulting from penetration of T. solium larvae into the central nervous system is the major cause of acquired epilepsy worldwide. [37] In 2000, about 39 million people were infected with trematodes (flukes) that naturally parasitize fish and crustaceans, but can pass to humans who eat raw or lightly cooked seafood. Infection of humans by the broad fish tapeworm Diphyllobothrium latum occasionally causes vitamin B12 deficiency and, in severe cases, megaloblastic anemia. [34]

    The threat to humans in developed countries is rising as a result of social trends: the increase in organic farming, which uses manure and sewage sludge rather than artificial fertilizers, spreads parasites both directly and via the droppings of seagulls which feed on manure and sludge the increasing popularity of raw or lightly cooked foods imports of meat, seafood and salad vegetables from high-risk areas and, as an underlying cause, reduced awareness of parasites compared with other public health issues such as pollution. In less-developed countries, inadequate sanitation and the use of human feces (night soil) as fertilizer or to enrich fish farm ponds continues to spread parasitic platyhelminths, whilst poorly designed water-supply and irrigation projects have provided additional channels for their spread. People in these countries usually cannot afford the cost of fuel required to cook food thoroughly enough to kill parasites. Controlling parasites that infect humans and livestock has become more difficult, as many species have become resistant to drugs that used to be effective, mainly for killing juveniles in meat. [34] While poorer countries still struggle with unintentional infection, cases have been reported of intentional infection in the US by dieters who are desperate for rapid weight-loss. [38]

    Pests Edit

    There is concern in northwest Europe (including the British Isles) regarding the possible proliferation of the New Zealand planarian Arthurdendyus triangulatus and the Australian flatworm Australoplana sanguinea, both of which prey on earthworms. [39] A. triangulatus is thought to have reached Europe in containers of plants imported by botanical gardens. [40]

    Benefits Edit

    In Hawaii, the planarian Endeavouria septemlineata has been used to control the imported giant African snail Achatina fulica, which was displacing native snails Platydemus manokwari, another planarian, has been used for the same purpose in Philippines, Indonesia, New Guinea and Guam. Although A. fulica has declined sharply in Hawaii, there are doubts about how much E. septemlineata contributed to this decline. However, P. manokwari is given credit for severely reducing, and in places exterminating, A. fulica – achieving much greater success than most biological pest control programs, which generally aim for a low, stable population of the pest species. The ability of planarians to take different kinds of prey and to resist starvation may account for their ability to decimate A. fulica. However, these planarians are a serious threat to native snails and should never be used for biological control. [41] [42]

    A study [43] in La Plata, Argentina, shows the potential for planarians such as Girardia anceps, Mesostoma ehrenbergii, and Bothromesostoma evelinae to reduce populations of the mosquito species Aedes aegypti and Culex pipiens. The experiment showed that G. anceps in particular can prey on all instars of both mosquito species yet maintain a steady predation rate over time. The ability of these flatworms to live in artificial containers demonstrated the potential of placing these species in popular mosquito breeding sites, which would ideally reduce the amount of mosquito-borne disease.

    More than meets the eye

    But there’s much more to flatworm vision than this. Gulyani and his colleagues next exploited the fact that their planarian flatworms can survive decapitation – and regrow their heads – to explore how they respond to light when headless.

    It turned out that the worms still reacted to light, but in the ultraviolet rather than the visible part of the spectrum. This suggests that the worms have evolved two completely independent ways to respond to light, say the researchers – one mediated through the eyespots and brain, and one a body-wide reflex that doesn’t involve the eyes, the exact mechanism for which still needs to be identified.

    Over the week-long period it took for the flatworms to regenerate their heads, the team monitored how quickly their brains and eyespots regrew, and when they began responding to visible light again.

    After four days, the eyespots had grown back, but the worms continued to react more strongly to UV than to visible light. Only after seven days did they regain their stronger preference to slither away from visible light – suggesting that their eyespots and brains were retaking control. It was not until the 12th day that their sensitivity to such light increased to the point that they reacted more strongly to light at the bluer end of the visible spectrum.

    Gulyani’s team speculates that the “gut instinct” response to UV light may be an ancient mechanism, with the eyespot and brain-controlled response to visible light a later evolutionary acquisition. As such, the researchers wonder whether their experiments might “replay” evolution in fast forward, showing how flatworms went from responding to ultraviolet light as an unthinking reflex to responding to visible light through a brain-controlled pathway.

    “It’s a fascinating coincidence that decapitation-regeneration experiments appear to copy – chronologically, at least – what may have occurred in evolution,” says Gulyani. It’s an idea that might be worth exploring in future experiments.

    Jochen Rink at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany, thinks the team’s experiment was beautifully designed, and creative in its use of planarians. “Where else in nature can you chop off a head and ask if the rest of the body can see light or not?” he says.

    Journal references: Science Advances, DOI: 10.1126/sciadv.1603025


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    A digestive tract, reproductive organs, and blood vessels could be seen inside the earthworm. The mouth led to a pharynx, esophagus (which was covered by the.

    5. Pathogenicity (remember it doesn’t need to be good at everything): Transmission Enters through the ingestion of contaminated water, soil, or plants as a.

    This stage also known as a caterpillar which cited by David (2004), caterpillar have hairy varying bristles or spike to prevent from getting eat and either b.

    Naegleria Fowleri Parasites are living organisms that survive by living and feeding off another organism. There are millions of different kinds of parasites.

    The mealworm has a spine but not the kind that we have. The mealworm is part of the invertebrate. The invertebrate family is a group of species that don’t ex.

    The Minnesota Department of Natural Resources (2016) reports that the zebra mussel ‘attaches and smothers’ native mussels. In order to thrive, this invasive .

    What should you tell Mr. Johnson to expect to see in cooper’s feces now that he has been dewormed? Is this parasite typically the cause of anemia’s? o The co.

    Background Strongyloides stercoralis is a soil-transmitted helminth that penetrates the skin as a method of infection. This parasitic worm gains nutrients a.

    The sea anemone gives the clownfish a home and protects the clownfish from predators while the clownfish can attract other fish into the sea anemone giving i.

    The crops are mad using Bacillus Thuringiensis, allowing the plant to produce “cry toxins” (Gene Watch). The idea behind this being a bad practice is that in.

    Planaria whole mount

    This specimen is stained to show the gastrovascular cavity, showing the small branches called diverticula. Note that there is one main branch of the gastrovascular cavity in the anterior part of the body, but two main branches posterior to the pharynx.

    The eyespots are simple and don't form an image that's why they are called eyespots instead of eyes. However, they are slightly cup-shaped and face toward the sides. With this arrangement, the flatworm can tell light from dark and move toward the dark.

    What came after flatworms?

    Probably around 545 million years ago, still in the Proterozoic period, not too long afterwards in terms of evolution, some of these flatworms evolved into roundworms.

    More about roundworms A roundworms project

    But there were still plenty of flatworms around too, and there still are, today. Some flatworms now live independently in the oceans, and some live in fresh water, while others live inside people and animals, both in the ocean and on land.

    Watch the video: Ένζυμα - βιολογικοί καταλύτες (May 2022).