Flatworm

Platyhelminthes
Fossil range: No fossils known
Bedford's flatworm, Pseudobiceros bedfordi
Bedford's flatworm, Pseudobiceros bedfordi
Scientific classification
Kingdom: Animalia
Subkingdom: Eumetazoa
Superphylum: Platyzoa
Phylum: Platyhelminthes
Gegenbaur, 1859
Classes

Monogenea
Trematoda
Cestoda
Turbellaria

The flatworms (Phylum Platyhelminthes from the Greek πλατύ, platy, meaning "flat" and ἕλμινς (root: ἑλμινθ-), helminth-, meaning worm) are a phylum of relatively simple, unsegmented[1] soft-bodied invertebrate animals. There are probably over 100,000 extant species,[1] making them the largest phylum of acoelomates. They are thought to have a basal position among the lophotrochozoa[2], although their phylogenetic position is currently very uncertain and the "phylum" may be paraphyletic.[1] Over half of all known flatworm species are parasitic.[1] They occupy a range of habitats, including marine, freshwater, and even damp terrestrial environments.

Some flatworms have serious negative effects on human health and ecosystems. A troublesome terrestrial example is the New Zealand flatworm, Arthurdendyus triangulatus, which rapidly colonized large areas of Ireland and Scotland, eating most of the indigenous earthworms.[3] There are four classes: Trematoda (flukes, probably paraphyletic), Cestoda (tapeworms), Monogenea, and Turbellaria.

Contents

Description

Distinguishing features

Platyhelminthes are bilaterally symmetrical animals, in other words their left and right sides are mirror images of each other; this also implies they they have distinct top and bottom surfaces and distinct head and tail ends. Like other bilaterians they have three main cell layers,[4] while the radially symmetrical cnidarians and ctenophore have only two cell layers.[5] Unlike other bilaterians, platyhelminthes have no internal body cavity and are therefore described as acoelomates. Thay also lack specialized circulatory and respiratory organs [4][6]

  Cnidarians[5] Ctenophores [5] Platyhelminthes [4][6] More "advanced" bilaterians[7]
Cnidocytes Yes No
Colloblasts No Yes No
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
Body cavity containing internal organs No Yes
Specialized circulatory and respiratory organs No Yes

The flatworm’s cephalized soft body is ribbon-shaped, flattened dorso-ventrally (from top to bottom), and bilaterally symmetric. Flatworms are the simplest triploblastic animals with organs. This means their organ systems form out of three germ layers: an outer ectoderm and an inner endoderm with a mesoderm between them. Turbellarians generally have a ciliated epidermis. There is also no true body cavity (coelom) except the gut; hence, flatworms are classified as acoelomates.The interior of the acoelomate body is filled with somewhat loosely spaced mesodermal tissue called parenchyma tissue.Flatworms have a hydrostatic skeleton, which consists of a water filled body cavity that is controlled by muscles.Flatworms exhibit an undulating form of locomotion.Depending on species and age, individuals can range in size from almost microscopic to over 20 m long. The longest ever recorded flatworm was a tapeworm over 90 ft (27 m) long.

Haptor

The haptor is a part of Platyhelminthes used to attach to its host. These differ between the prohaptor which is on the anterior end of the body and the opisthaptor, which is part of the posterior end of the body.The prohaptor has various adhesive and feeding structures. In some species, the prohaptor may have a number of cephalic or head glands that secrete a sticky adhesive substance, and shallow muscular suckers, all used for attachment. In other species there is an oral sucker, with various degrees of muscularisation that surrounds the mouth. The opisthaptor is primarily responsible for the attachment of the monogeneans to the host. The morphology of the opisthaptor is highly variable. It may have suckers in various degrees of development, large hooks called anchors (or hamuli), small hooks that are remnants from the larval stage, or complex clamps that may be either muscular or sclerotised.

Circulation and nervous system

There is no true circulatory or respiratory system, but like all other animals, flatworms do take in oxygen. Extracellular body fluids (interstitial fluids) percolate between cells to help distribute nutrients, gases, and waste products.

Flatworms respire at their integument; gasses diffuse directly across their moist outer surface. This type of system is called integumentary exchange.

However, flatworms do have a bilateral nervous system; they are the simplest animals to have one. Two cordlike nerves branch repeatedly in an array resembling a ladder. The head end of some species even has a collection of ganglia acting as a rudimentary brain to integrate signals from sensory organs such as eyespots.

Vision

Where present, flatworms have simple, occellus type eyes on the upper surface of their body.[2] They may just have eyespots; when their eyes regenerate (if lost) they form eyespots before developing into fully fledged ocelli.[2] Of course, eyes are of no utility to many parasitic worms and have been secondarily lost in these lineages. The nervous system supporting flatworms' eyes is very similar to that of other invertebrates, suggesting a common origin of the ocellus-type eye.[2]

Feeding

Species of flatworm have a wide range of feeding habits; some are predatory, others deposit feeders; others are symbiotic whereas others are outright parasitic.[1]

Usually the digestive tract has one opening, so the animal can feed, digest, and eliminate undigested particles of food simultaneously, as most animals with tubular guts are able to do. This blind-ended gastrovascular cavity functions similarly to that of the Cnidaria. However, in a few particularly long flatworms or those with highly branched guts, there may be one or more anuses. A small group where the gut is absent or non-permanent, called acoel flatworms, appear to be unrelated to the other Platyhelminthes (see below). They eat rarely, maybe a few times per year (once or twice). Their diet includes small bugs found in soils and other types of worms, smaller than itself. Despite the simplicity of the digestive chamber, they are significantly more complex than cnidarians in that they possess numerous organs, and are therefore said to show an organ level of organization. Mesoderm allows for the development of these organs, and true muscle. Major sense organs are concentrated in the front end of the animals for species who possess these organs. Muscular contraction in the gut causes a strong sucking force which allows flatworms to ingest food.

Reproduction

Flatworm reproduction is hermaphroditic, meaning each individual produces eggs and sperm. When two flatworms mate, they exchange sperm so both become fertilized. Some flatworms, such as Pseudobiceros hancockanus engage in penis fencing, in which two individuals fight, trying to pierce the skin of the other with their penises; the first to succeed inseminates the other, which must then carry and nourish the fertilized eggs.[8] Flatworms usually do not fertilize their own eggs.

Turbellarians classified as planarians (usually freshwater, non-parasitic) can also reproduce asexually by transverse fission. The body constricts at the midsection, and the posterior end grips a substrate. After a few hours of tugging, the body rips apart at the constriction. Each half grows replacements of the missing pieces to form two whole flatworms. This also means that if one of these planarian flatworms is cut in half, each half will regenerate, forming two separate, fully-functioning flatworms.

Classification

"Platodes" from Ernst Haeckel's Kunstformen der Natur, 1909

Flatworms were formerly considered to be basal among the protostomes. Molecular evidence suggests that this is only true of the orders Acoela and Nemertodermatida, which are thus given their own phylum Acoelomorpha. These findings, however, are still not accepted by all biologists. The systematic position of Catenulida seems uncertain, although Donoghue and Cracraft would place it as a sister group to all other non-Acoelomorpha flatworms.[9] Xenoturbella was at first believed to be a flatworm as well, but it is now obvious that it belongs in its own phyla. The remaining and true flatworms form a monophyletic group that developed from more complex ancestors, and grouped with several other phyla as the Platyzoa. The traditional classifications of flatworms is primarily based on differing degrees of parasitism and divided into three monophyletic classes:

The remaining flatworms are grouped together for convenience as the class Turbellaria, now comprising the following orders:

Most of these groups include free-living forms. The flukes and tapeworms, though, are parasitic, and a few cause massive damage to humans and other animals.

Fossils

The platyhelminthes, being soft bodied, are not prone to fossilisation. The only preserved instances known have been retrieved from Egyptian mummies.[1]

Biochemical memory experiments

In 1955, Thompson and James V. McConnell 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. Incredibly these 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 perhaps 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 its behaviour.[10] 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.[11] No double-blind experiment has ever reproduced his results.

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Klaus Rohde (2001), "Platyhelminthes (flatworms)", Encyclopaedia of Life Sciences, doi:10.1038/npg.els.0001585 
  2. ↑ 2.0 2.1 2.2 2.3 Saló, E (2002). "Genetic network of the eye in Platyhelminthes: expression and functional analysis of some players during planarian regeneration". Gene 287: 67. doi:10.1016/S0378-1119(01)00863-0. 
  3. ↑ Boag, B, K A Evans, G W Yeates, P M Johns & R Nielson (1995). "Assessment of the global potential distribution of the predatory land planarian Artioposthia triangulata (Dendy) (Tricladida: Terricola) from ecoclimatic data". New Zealand Journal of Zoology 22: 311–318. http://www.rsnz.org/publish/nzjz/1995/76.pdf. 
  4. ↑ 4.0 4.1 4.2 Hinde, R.T., (2001). "The Platyhelminthes". in Anderson, D.T.,. Invertebrate Zoology. Oxford University Press. pp. 58–80. ISBN 0195513681. 
  5. ↑ 5.0 5.1 5.2 Hinde, R.T., (2001). "The Cnidaria and Ctenophora". in Anderson, D.T.,. Invertebrate Zoology. Oxford University Press. pp. 28–57. ISBN 0195513681. 
  6. ↑ 6.0 6.1 Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 226–269. ISBN 0030259827. 
  7. ↑ Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 196–224. ISBN 0030259827. 
  8. ↑ Leslie Newman. "Fighting to mate: flatworm penis fencing". PBS.
  9. ↑ Donoghue, Michael J. and Joel Cracraft. (2004): Assembling the Tree of Life. Oxford University Press. ISBN 0195172345 P. 213. On-line at Google Books
  10. ↑ Bob Kentridge. "Investigations of the cellular bases of memory". University of Durham. Retrieved on 2007-02-08.
  11. ↑ Rilling, M. (1996). "The mystery of the vanished citations: James McConnell's forgotten 1960s quest for planarian learning, a biochemical engram, and celebrity.". American Psychologist 51: 589–598. doi:10.1037/0003-066X.51.6.589. 

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