Here is a term paper on ‘Platyhelminthes’. Find paragraphs, long and short term papers on ‘Platyhelminthes’ especially written for college and medical students.
Term Paper # 1. Introduction to Platyhelminthes:
Platyhelminthes (Gr. Platys, flat + helmins, worm) are acoelomate; triploblastic, bilaterally symmetrical, vermiform worms. They show low organization as they are without anus and without skeletal, respiratory and circulatory systems. The body is filled with mesenchymal cells which are mesodermal in origin.
The flatworms are said to have evolved from a coelenterate like ancestor which changed from radial to bilateral symmetry during evolution. Trematoda seems to have evolved from some commensal or parasitic rhabdocoel turbellarians and the cestodes must have evolved independently from the trematodes.
Term Paper # 2. General Characters of Platyhelminthes:
(a) They are bilaterally symmetrical and dorso-ventrally flattened. Some are small leaf-like or long ribbon shaped.
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(b) Flatworms are triploblastic having all the three germ layers, viz. ectoderm, mesoderm and endoderm.
(c) Most of them are white or colourless but some free-living ones are brown, preys, black or have other brilliant colours.
(d) Body is covered with a cellular syncytial partly ciliated epidermis but in some parasitic forms the epidermis is absent and cuticle is present. Exo and endo-skeletons are absent.
(e) Adhesive structures like hooks, spines and suckers are present in parasitic forms.
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(f) Mesodermal muscular system consists of longitudinal, circular and oblique muscle beneath the epidermis.
(g) They are acoelomate as true coelome is absent. The space between body organs is filled with parenchyma.
(h) Digestive system is absent in Acoela and Cestola but in others it consists of mouth, pharynx and blind intestine as there is no anus.
(i) Respiratory and circulatory systems are absent.
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(j) Excretory system consists of protonephridia with flame cells and bulbs.
(k) The nervous system consists of a pair of cerebral ganglia one to three pairs of longitudinal nerve cords connected to each other by transverse commissures. Sense organs are present in the form of chemo and tango-receptors such as ciliated pits and grooves in free-living forms but are reduced in parasitic forms.
(l) Most of them are hermaphrodite with complex reproductive system having well developed gonads, gonoducts and accessory organs. Eggs mostly without yolk but provided with shell and yolk glands.
(m) Fertilisation is internal and mostly self-fertilisation and sometimes cross-fertilisation takes place.
(n) Life cycle is complicated, and usually in two hosts. There may be many types of larvae. Parthenogenesis also takes place in some cases. They may also propagate by endogenous budding.
(o) They may be free-living, commensals or parasitic.
Term Paper # 3. Classification of Platyhelminthes:
Phylum Platyhelminthes is divisible into three classes:
Class—I. Turbellaria:
1. Mostly free living flatworms, some are ecto or endo-commensals or parasites.
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2. Body soft, un-segmented, dorso-ventrally flattened and covered with ciliated, cellular or syncytial epidermis, having secretory cells and rod-shaped bodies or rhabdites.
3. Digestive system consists of mouth, pharynx and intestine.
4. Excretory system with protonephridia and flame bulbs.
5. Mostly hermaphrodite, reproduction, asexual, sexual and by regeneration.
6. They may be terrestrial, marine or fresh water.
Class turbellaria is divisible into 5 orders:
Order—1. Acoela:
1. Small turbellarian, less than 2 mm long.
2. Digestive system consists of mouth and simple pharynx but no intestine.
3. Excretory system is absent, reproductive system is without oviducts and yolk good.
4. All are marine, mostly free-living found under stones, algae or on bottom mud. Some harbour intestine or see urchins and see cucumbers.
Example – Convoluta.
Order—2. Rhabdocoela:
1. Small turbellarians, measuring less than 3 mm.
2. Digestive system consists of mouth, simple pharynx and un-branched sac-like intestine.
3. Excretory system consists of protonephridia.
4. Two ventral longitudinal nerve cords.
5. One or two compact gonads, oviducts and yolk glands constitute the reproductive system.
6. They may be marine, fresh water, terrestrial. Some are free-living, others may be commensal or parasitic.
Examples – Stenostomum, Mesostoma, etc.
Order—3. Allocoela:
1. Usually small, measuring 1 to 10 mm.
2. Digestive system consists of simple pharynx sometime bulbous or plicate and straight or branched intestine.
3. Excretory system consists of paired protonephridia with two or three branches and nephridiopores.
4. Nervous system with 3 or 4 pairs of longitudinal trunks provided with transverse commissures.
5. Reproductive system consists of numerous testes and a pair of ovaries. Fenis papilla is mostly present.
6. Mostly marine, beings commonly found in littoral sand and mud. Some are fresh-water.
Examples – Prorhynches etc.
Order—4. Tricladida:
1. Large turbellarians, measuring from 2 to 60 cm in length.
2. Digestive system includes mouth, plicate pharynx and intestine with three highly diverticulated branches.
3. Excretory system consists of protonephridia with many nepridiopores.
4. Eyes are mostly present.
5. Two or numerous testes and a pair of ovaries with yolk glands. A penis papilla is present. Single gonopore.
6. They may be marine, freshwater or terrestrial.
Examples – Dugesia, Gundu, Bipalium, etc.
Order—5. Polycladida:
1. Body usually broad and flattened. Some are long and are of moderate size, 2—20 mm.
2. Pharynx is plicate opening into branched intestine.
3. Well-developed many nerve cords and numerous eyes.
4. Numerous, scattered testes and ovaries, yolk glands are absent. Separate gonopores.
5. Exclusively marine.
Examples – Leptoplana, Thysanozoon, Notophana, etc.
Class—II. Trematoda:
1. Ecto or endo-parasitic flukes.
2. Body is usually un-segmented, dorso-ventrally flattened and leaf-like.
3. Body covered with cuticle and no cilia. Well-developed suckers and sometime hooks are present.
4. Digestive system consists of mouth, muscular pharynx and two branches of intestine. Anus is absent.
5. Excretory organs consist of protonepridia.
6. There are 3 pairs of longitudinal nerve cords.
7. Most are hermaphrodites. There is single ovary and two to many testes. Life cycle may be simple or complicated with more than one host.
The class is divisible into three orders:
Order—1. Monogenia:
1. Ecto or endo-parasites in cold-blooded aquatic vertebrates.
2. Oral sucker if present is weak. Anterior and is provided with a pair of adhesive structures.
3. There are two excretory pores situated anteriorly on the dorsal side.
4. Reproductive system consists of separate male and female genital pores. There is one or two vagina and small uterus with a few shelled eggs.
5. Free swimming larva is known as onchomiracidium.
Examples – Gyrodactylus, Polystoma, Diplozoon etc.
Order—2. Digenea:
1. Endoparasitic in vertebrate and invertebrate hosts. They have more than one host in the life cycle.
2. There is an oral sucker around the mouth and a ventral sucker. Both suckers are without hooks.
3. Single excretory pore at the posterior end.
4. Vagina is absent. Uterus is long and branched containing shelled eggs.
5. Life cycle is complicated having different types of larvae and one or more intermediate hosts.
Examples – Fasciota, Schistosoma, Echinotoma etc.
Order—3. Aspidobothrea:
1. Endoparasite in the digestive tract of fishes and reptiles.
2. Oral sucker is absent. The ventral surface is covered with an adhesive organ.
3. Single, nephridiopore.
4. Only one testis and the life cycle is simple with only one host.
Example – Aspidogaster.
Class—III. Cestoda:
1. Endoparasite in the intestine of vertebrates.
2. Long, flattened, ribbon-like body, hence known as tapeworms.
3. Body is without epidermis and cilia but a cuticle is present.
4. Body is usually divided into head or scolex and few to many segments or proglottids. Scolex is provided with suckers, hooks or both.
5. No mouth or digestive tract. Excretory system consists of protonephridia with flame cells.
6. Nervous system consists of a pair of ganglia and two lateral longitudinal nerves.
7. Each proglottid contains one or two sets of male and female reproductive organs.
8. Life cycle is complicated with hooked embryo and is passed in more than one host.
The class is divided into two sub-classes:
Sub-Class—A. Cestodaria:
1. Endoparasites in the intestine and coelomic cavities of fishes and reptiles.
2. There is no scolex and body is not segmented.
3. Only a single set of reproductive organs present.
4. Larva with 10 hooks.
The sub-class is divided into two orders:
Order—1. Amphilinidea:
1. Endoparasites in the coelom of fishes.
2. Body is flattened, oval or elongated. Suckers are absent. A protrusible proboscis and frontal glands are present at the anterior end.
3. Long and much coiled uterus opens near the anterior end.
Example – Amphilina.
Order—2. Gyrocotylidea:
1. Endoparasite in the intestine of fishes.
2. Long, flattened body with an anterior sucker and a posterior rosette-shaped adhesive organ. Eversible proboscis present at the anterior end.
3. Uterine, male and vaginal pores are situated together in the posterior half, of the body. Uterus is straight and extends directly to the pore.
Example – Gyrocotyle.
Sub-Class—B. Eucestoda:
1. Body is usually long and ribbon-like. It is divided into scolex, neck and strobila with many proglottids. The scolex bears adhesive structures.
2. Each proglottid has more than one set of complete reproductive organs.
3. Six hooked larva.
4. Endoparasite in the digestive tract of vertebrate host.
The sub-class is divided into five orders:
Order—1. Terraphyllidea:
1. Endoparasites in the intestine of cartilaginous fishes.
2. Scolex bears four leaf-like out-growth known as bothridia often provided with hooks.
3. Testes lie in front of ovaries.
4. Vitelline glands are scattered in two lateral bands.
5. Cirrus is armed with hairs, spines or hooks.
Example – Phyllobothrlum.
Order—2. Diphyllidea:
1. Parasite in the intestine of cartilaginous fishes.
2. Strobila has less than 20 segments only.
3. Scolex has two bothridia and a spiny head stalk.
Example – Echinobothrium
Order—3. Trypanorhyncha:
1. Parasitic in the spiral valve of the digestive tract of cartilaginous fishes.
2. Strobila is segmented and is of moderate size.
3. Scolex with two or four sessile bothria and four protrusifle spiny proboscis.
4. Testes extend beyond ovary posteriorly.
Example – Tetrarhynchus.
Order—4. Pseudophyllidea:
1. Endoparasite in vertebrate intestine.
2. Body is usually segmented, sometimes un-segmented.
3. Scolex has two to six bothria or suckers and mostly with adhesive organs.
4. Numerous yolk glands are present.
Example – Bothriocephalus.
Order—5. Taenioidea:
1. Endoparasite in the intestine of birds and mammals.
2. Elongated body, segmented into proglottids.
3. Scolex bears four suckers often with an apical rostellum provided with hooks.
4. Excretory system with four longitudinal vessels and flame cells.
5. Complete sets of reproductive organs in each proglottid. Single compact yolk gland.
6. More than one host.
Examples – Taenia, Echinococcus.
Term Paper # 4. Representatives of Platyhelminthes:
1. Convoluta:
It is small, marine animal with gregarious habit, found under stones, among algae between tide zones. Body has characteristic patches formed by the groups of pigment cells. There is a cluster of frontal glands at the anterior end and a pair of eyes and a statocyst. Mouth is ventral through which pharynx protrudes during feeding.
Nervous system is represented by few pairs of nerve cords which are connected by cross strands. No excretory system. It is hermaphrodite with protrandrous condition. Both male and female genital pores are situated close to each other ventrally. Gonads are not differentiated. Convoluta shows symbiosis by having algal cells in the body.
2. Bipallium:
Bipallium is commonly found is green houses throughout the world. The body is long, being about 50 cm in length. The body is divisible into head and body proper. The head bears a row of many eyes along its margin. The animal divides asexually by fragmentation and also breeds sexually.
3. Thysanozoon:
Thysanozoon is found commonly in a less cold water of sea. Body is covered with numerous papillae each containing an intestinal branch. Anterior end bear a pair of marginal tentacles and numerous cerebral eyes. Pharynx is tubular. Glandulo-musclar adhesive organ is present on the dorsal surface behind the female gonopore. Hermaphroditic is with a pair of male pore and single female pore. Seminal bursa is absent.
4. Gyrodactylus:
Gyrodactylus is an ectoparasitic on the gills and skin of freshwater fishes. Body is minute and elongated. Anterior sucker absent but anterior end is provided with adhesive organs and adhesive glands. Eyes are absent. Intestine is saclike forked into two branches without diverticula. The opisthaptor is disc-shaped and provided with one pair of anchors (hooks) and 16 marginal hooks (hooklets). Genito-intestinal canal is absent. Genital pore is median. Gyrodactylus is viviparous.
5. Aspidogaster:
Aspidogaster lies in the pericardial and renal cavities of freshwater mussel and in the gut of fishes and turtles. A large sucker occupying greater portion of the ventral surface is present. The sucker is subdivided into four longitudinal rows of sucking cups or alveoli. The narrow anterior end has a sub-terminal mouth which is devoid of anoral sucker.
The gut is simple, straight and sac-like. Excretory system consists of protonephridia with separate excretory bladders. Hermaphroditic. Single testis and folded unpaired ovary. Life cycle is simple without alternation of host.
6. Diplozoon:
The flatworm is ectoparasite on the gills of fresh water fishes. The characteristic feature is the fusion of two larvae during the life history and remains in the fused condition throughout life. The pair seems to be in the state of permanent copulation. Each individual bears an oral sucker anteriorly and a pair of lateral suckers. A little behind are present four pairs of suckers ventrally at the posterior end.
There is a muscular pharynx and un-forked intestine having numerous diverticulae. There is a single testis and single ovary in each individual. The uterus is surrounded by Mehlis gland and has a single egg. The vitelline gland has numerous follicles.
7. Polystomum:
Polystomum is an endoparasite of the urinary bladder of frogs and turtles.
It is leaf-life, dorso-ventrally flattened. At its posterior end is an opisthaptor having six suckers in a circle and several small chitinous hooks and two large curved hooks or anchors. At its anterior end is an oral sucker enclosing the mouth. Intestine is bifurcated connected by transverse connectives. It is unisexual.
Fertilization is internal. The fertilized eggs escape out with the faeces and develop into larvae. Each larva has an eye, a large posterior sucker and cilia arranged in five circular bands around the body. These larvae are attached to internal gills of tadpoles, drop off their cilia and reach into the urinary bladder. These they become sexually mature in three years.
8. Diphylidium Caninum:
It is commonly known as dog tapeworm. It has world-wide distribution. It makes its presence in the intestine. It measures 15-40 cm in length and has under 200 proglottids. The scolex is somewhat triangular and bears four deeply cupped suckers.
Rostellum is retractile into a rosteller sac in scolex. It bears about 60 hooks in 3-7 rings. Mature proglottids are long and barrel shaped having a double set of male and female sex organs. The genital pares at the middle of both lateral margins. Numerous testers are present before or behind the female genitalia.
9. Maniezia:
It makes its presence in sheep catties and other ruminants. Body may be divided into scolex, neck and proglottids. The scolex is small with prominent suckers. The rostellum and hooks are absent. Each proglottid is broader than long and contains a double set of reproductive organs. Genital ducts are dorsal to osmo-regulatory canals.
Testes are numerous scattered throughout the centre of proglottid. Citrus in cirrus pouch. Ovaries are in the form of an open fan. Uteri form networks. On one side vagina is dorsal to cirrus pouch and on other ventral. The uterus develops as a network of canals, but latter breaks up into a large number of uterine capsules, each enclosing 3-30 embryos.
10. Diphyllobothrium (Dibothriocephalus):
It is the largest and most pathogenic cestode of human beings. It occurs in intestine and worldwide in distribution. It measures 20 metres in length. Body may be divided into scolex, neck and proglottids. The number of proglottids is 3000-4000. Scolex is fusiform and bears two slit like suckers or bothria. Mature proglottid is broader than long.
Each proglottide is hermaphrodite and contains complete sets of male and female reproductive organs. Testes are numerous vasa efferentia unite to form coiled sperm duct. Ovary is trilobed. Vagina runs posteriorly forming seminal vesicle. Uterus very much twisted.
Gonopores ventral life cycle involves two intermediate hosts, one is Cyclops and other a fish. It causes bothriocephalus anaemia, erythropenia and haemorrhage.
11. Gyrocotyle:
Gyrocotyle is found in the intestine of chimaeroid fishes. Body is elongated flattened with a ruff, the rosette, at one end surrounding funnel-shaped depression. The anterior end bears a large opening that leads into a highly muscular protrusible mass, the proboscis.
The margins of the body are generally ruffled. Hermaphroditic. Male system consists of scattered testes, sperm duct and penis papilla, while female system comprises ovary, oviduct and uterus. Yolk glands consist of follicles scattered throughout most of the body.
12. Fasciolopsis Buski:
It is a large intestinal fluke of man, dog and pigs, quite common in India and China. It measures 20-75 mm in length and 8-20 mm in breadth. The ventral sucker is larger and close to anterior sucker. Pharynx is small, oesophagus short and caeca long and non-diverticulate. Gonads occupy posterior region of the body. In general it resembles to Fasciola hepatica. Miracidium larva comes out of egg after three weeks of egg laying.
Intermediate host is a snail (Planorbis or Segmentina). Cercaria encysts on tubers of water caltrop or Trapa (Singhara), which are sold in the market. They are eaten raw and in this way infection takes place. Infected human beings suffer from inflammation and haemorrhage of the intestine, which may also develop ulcers. It is treated by the use of oil of Chenopodium, Hexylresorcinol and Carbon tetrachloride, etc.
13. Paragonimus:
Paragonimus is a common lung fluke parasitizing carnivorous mammals, including man producing paragonimiasis. It is a serious parasite of man in China, Japan, Korea. New Guinea, Philippines, Peru, Yucatan, Africa, India and parts of U.S.A. It also occurs in cats, dogs, pigs, goats, rats and various wild mammals, showing little host specificity. The first species to be described was P. westermani from Bengal tigers. The first species found infecting man was named P. ringeri. Whether these two species are distinct or the same is not certain.
The adult lung-fluke is oval in shape, reddish brown in colour, about 8 to 20 mm long and 4 to 6 mm in diameter. The cuticle bears numerous minute spines. The adult lives in the lung in cyst-like pockets which rupture to liberate the eggs into the branchial tubes of the host.
The life history involves two intermediate hosts. Eggs discharged in sputum or faeces hatch into free-swimming miracidia that enter the first intermediate host, a suitable amphibious snail. After passing through sporocyst and two generations of rediae, the cercariae leave the snail and enter the second intermediate host, usually a freshwater crab or crayfish, to encyst in their soft tissues.
Man is infected by eating poorly-cooked crabs and crayfish or by drinking water containing larvae freed from dead crustaceans. When eaten the cysts are dissolved and the young flukes enter various organs of the human body and finally mature in the lungs.
Infected human beings suffer from intermittent cough, fever, streaks of blood in sputum, anaemia, weakness and sometimes tuberculosis. Infection is treated by the use of Emetine hydrochloride.
14. Echinococcus:
It is a minute tapeworm 3 to 6 mm long. It is common in Africa, Australia, New Zealand, Iceland, Holland and South America. It makes its presence in the intestine of dogs, cats and wolves. The scolex bears four suckers and a protrucible rostellum with two rows of hooks. There is a neck and three or four proglottids, one immature, 1 or 2 mature with complete hermaphrodite reproductive organs, and one large gravid proglottids.
The eggs pass out with the faces of the host and gain access to an intermediate host which is man or herbivorous animals like rabbits, kangaroos, sheep and cattle in which the shell is dissolved and the six hooked embryos bores its way in the liver or lungs or sometimes into kidneys, spleen, bones, heart and brain. The young larva changes into a hollow bladder around which the host forms an enveloping fibres cyst wall. It is now called a hydatid cyst.
The hydatid cyst has a colourless fluid which may be from 2 to 50 quarts, but older cysts have a granular deposit consisting of broad capsules and free scolices. The scolices are finally evaginated in broad capsules and in endogenous and exogenous daughter cysts and should they reach the final host, a dog can or wolf, they, develops into adult Echinococcus.
15. Tetrarhynchus:
Tetrarhynchus is an endoparasite in the intestine of elasmobranch fishes. Body is long, divisible into scolex and proglottids and broader at the posterior end. Scolex is elongated and divisible into a long proximal part containing the proboscis apparatus and a distal part bearing the bothria. Scolex bears four bothria (suckers) each with an eversible proboscides armed with spines. Proboscides or rostella are enclosed in muscular sheaths which end below in muscular bulbs. Reproductive system is single in each proglottid.
16. Amphilina:
Amphilina is monozoic tapeworm under sub-class Cestodaria. It is found in the body cavity of European sturgeon namely Acipenser. The body consists of a single individual. It is flat, and leaf-like. The anterior end contains a protrusible proboscis. Vagina lies behind the ovary and opens on the left side of the body about 2 mm from the male aperture.
Testes are scattered and cirrus is armed. The eggs are swallowed by the crustacean intermediate host, Gammarus, which is eaten by a sturgeon and thus the life-cycle is completed.
17. Dibothriocephalus:
Dibothriocephalus latus, (formerly Diphyllobothrium latum) is the largest and most pathogenic (injurious) cestode to mankind measuring 20 meters in length with 3,000-4,000 proglottids. It is commonly called as “broad” of “fish-tapeworm”. It is found in the small intestine of carnivores such as the cats, dogs, polar bears, seals and sea-lions, including man, and has a world-wide distribution.
Scolex is fusiform, bears two slit-like bothria, followed by the long slender neck. Mature proglottid is broader than long. Excretory system is protonephridial type. Hermaphroditic. Male system consists of numerous testes, vas deferens, seminal vesicle and cirrus.
Female system consists of a bilobed ovary, oviduct and a coiled uterus. Each segment has a single set of reproductive system. Life cycle involves two intermediate hosts, one is copepod (Cyclops) and another fish. Infection in man results by eating insufficiently cooked infected fish.
18. Taenia Saginata (Beef Tapeworm):
Taenia saginata is the mostly common of the large human tapeworms. It has a cosmopolitan distribution, particularly prevalent in beef eaters. It is commonly called as the beef tapeworm because the larvae usually occur in the flesh of cattle. The adult worm lies attached in the mucosa of the intestine of man.
Taenia saginata is much larger than Taenia solium and may attain a length of 4 to 12 meters (even 25 meters), longer than the intestine in which it lives. The scolex measures 1.5 to 2 mm in diameter and possesses four symmetrically arranged hemispherical suctorial suckers serving as sole organs of adhesion. But the rostellum and hooks are lacking.
The strobila may consist of up to 2,000 proglottids, producing from 8 to 10 proglottids daily. Mature proglottids are broader than long. Testes are numerous, 300 to 400 in number. The numerous vasa efferentia unite to form a common vas deferens or sperm duct, which is distally modified into cirrus opening into the genital atrium.
Ovary is in the form of two lobes. Vitelline glands are compact and situated behind the ootype. Ootype is surrounded by Mehlis’s glands. In gravid proglottids, uterus becomes branched and genital organs atrophy. The gravid proglottids, passing out through anus, are white or yellowish, rather active and each about 2.5 cm in length.
The intermediate hosts and cattle and buffaloes. Life history is like that of T. solium. The onchospheres finally lodge in the heart of muscles where they form cysts and develop into cysticerci, called Cysticercus bovis. Man acquires this tapeworm by eating infected, poorly cooked beef containing cysticerci, which can be killed by sufficient cooking or by quick freezing of beef. The incidence of T. saginata is higher than T. solium.
In the Dog and Cat Tapeworm, Taenia pisiformis, the adults are found in dogs and cats and the larvae in liver and mesenteries of rabbits.
19. Hymenolepis:
Hymenolepisnana commonly known as dwarf tapeworm is an endoparasite in the intestine of man. It is the smallest tapeworm measuring 7 to 100 mm in length. Body consists of a scolex neck and proglottids. Scolex bears a well-developed retractile rostellum with a crown of 20-30 hooks. Neck is slender and long. Proglottids are usually broader than long. Life cycle is completed in a single host. No intermediate host is required. Hymenolepisnana causes severe toxic symptoms including abdominal pain and diarrhoea, etc. It is worldwide in distribution. It is the commonest tapeworm in Southern United States.