Phylum Platyhelminthes

The phylum Platyhelminthes was coined by Gegenbaur in 1859. It includes the simplest animals that are bilaterally symmetrical and triploblastic (composed of three fundamentals

cell layers). These are also known as the flatworms. Flatworms have no body cavity other than the gut and lack an anus; the same pharyngeal opening both takes in food and expels

waste. Because of the lack of any other body cavity, in larger flatworms the gut is often very highly branched in order to transport food to all parts of the body. The lack of a cavity also constrains flatworms to be flat; they must respire by diffusion, and no cell can be too far from the outside, making a flattened shape necessary.

Flatworms are once divided into three groups. The free-living Turbellaria include the

planarian dugesia, shown above; these are found in the oceans, in fresh water, and in moist terrestrial habitats, and a few are parasitic. The Trematoda, or flukes, are all parasitic, and have complex life cycles specialized for parasitism in animal tissues. Members of one major taxon of flukes, the Digenea -- which includes the human lung fluke which pass through a number of juvenile stages that are parasitic in one, two, or more intermediate hosts before reaching adulthood, at which time they parasitize a definitive host. The Cestoda, or tapeworms, are intestinal parasites in vertebrates, and they also show anatomical and life history modifications for parasitism. Platyhelminths have practically no fossil record. A few trace fossils have been reported (Alessandrello et al., 1988), and fossil trematode eggs have been found in Egyptian mummies and in the dried dung of Pleistocene ground sloth.

General Characters of Platyhelminthes

Bilaterally symmetrical with definite polarity of anterior (head) and posterior (tail) ends
Triploblastic, i.e., body derived from three embryonic germ layers: ectoderm, mesoderm and endoderm.
Dorso – ventrally flattened.
Usually with a well- defined ventral surface bearing mouth and gonophores.
Free- living commensal or parasitic forms.
Tissue- organ grade of organization, i.e., body cells aggregate into definite tissues and tissues make up organs.
Body unsegmented (except in class Cestoda).
Acoelomate, i.e., without any body cavity or true coelom.
Spaces between various organs filled with special mesodermal tissue, the mesenchyme or parenchyma.

Adhesive structures like hooks, spines and suckers, and adhesive secretions common in parasitic forms.
Epidermis cellular or syncytial, frequently ciliated. Absent in some.
Muscular system of mesodermal origin.
Longitudinal, circular and oblique muscle layers beneath epidermis.
Digestive system branched and incomplete without anus. Altogether absent in Acoela and Cestoda.
Skeletal, respiratory and circulatory system are wanting.
Excretory system includes lateral canals and proto-nephridial (flame cells). Absent in some primitive form.
Nervous system primitive, ladder- like. Comprises a pair of anterior ganglia with longitudinal nerve cords connected by transverse nerves.
Sense organs simple. Eye- spots or photo receptors in free living forms.
Mostly monoecious (hermaphrodite) with complex reproductive system.
Well- developed gonads, gonoducts and accessory organs.
Eggs mostly devoid of yolk.
Yolk produced separately in yolk or vitelline glands.
Fertilization internal may be cross or self.
Development direct or indirect.
Usually indirect in endoparasites with a complicated life cycle involving many larvae and hosts.

Cercaria larva: -

A fully formed cercaria larva possesses a flattened heart-shaped body with a long contractile tail. Its body surface covered with thin cuticle with backwardly directed spines. The anterior end bears mouth, muscular pharynx, esophagus and bifid intestine. The mouth is surrounded by oral sucker. A ventral sucker or acetabulum is also present between the two limbs of the intestine. There are numerous flame cells. A small excretory duct arises from the bladder and opens to the exterior by excretory pore situated at the base of the tail. A number of unicellular cystogenous glands are situated below the large body wall.

Their secretion forms the cyst around the larva when it is converted into metacercaria. Groups of germ cells are also present. When mature the cercaria leaves the Redia through birth pore it also wriggles out of the snail body. It swims in water for some time and finally settles down on the blade of some aquatic weed. It sheds off the tail and a cyst are formed by the secretion of cystogenous gland. Thus, a metacercaria is formed.

Metacercaria: -

The metacercaria is somewhat rounded with thick outer covering of cuticle in the form of

cyst. The Cystogrenous cells of the cercaria disappear and the flame cells increase in number.

Important point: -

The phylum name was coined by Gegenbaur in 1859. The simplest animals that are bilaterally symmetrical and triploblastic (composed of three fundamental cell layers) are the Platyhelminthes, the flatworms. Flatworms were once divided into three groups.
A. Class - Turbellaria B. Class- Trematoda C. Class- Cestoda.The class -Turbellaria includes all free-living members of the phylum, as well as a few parasites. It includes many marine forms, whose beautiful colors serve as a warning of their toxicity to would be predators, as well as the more drab freshwater planarians.
The class-Trematoda, commonly called flukes, are unsegmented parasitic flatworms that usually parasitize a snail as an intermediate host (in which they reproduce asexually) and a human or other vertebrate as a definitive host (in which the worm's mate and lay eggs).
The class-Cestoda, commonly called tapeworms, are segmented, ribbon like parasites usually found as adults in the small intestines of vertebrate animals.
Platyhelminths have practically no fossil record. A few trace fossils have been reported that were probably made by platyhelminths (Alessandrello et al., 1988), and fossiltrematode eggs have been found in Egyptian mummies and in the dried dung of Pleistocene ground sloth.
Fasciola hepatica is a common liver fluke.
Main species of Fasciola like Fasciola hepatica (Liver fluke- Sheep) and Fasciola gigantica (Liver fluke – Cattal).

Fasciola hepatica is a digenetic trematode.
It is endo-parasite in the bill passage and liver of sheep and cattle and completes part of its life cycle in snail.
Its primary host is sheep and secondary host is freshwater snail.
Shape: - Body is soft and fleshly. It is Dor-ventrally flattened and leaf like, somewhat oval in appearance.
Size: - It is about 18-30mm long and 4-15mm broad.
Colour: - The colour is slightly pinkish and the margins appear to be brownish or blackish. Internal organs are roughly visible because of transparent body wall.
The eggs are fertilized in the uterus or in ootype. Fasciola is a hermaphrodite animal, but cross fertilization occurs.
The fully formed fertilized capsulated eggs are inside the uterus. The first cleavage is complete but unequal and produced: -
(i) a small granular propagatory cell and
(ii) a large somatic or ectoderm of larva. Segmentation and cleavage start even when the eggs are inside the uterus.
The first cleavage is complete but unequal and produces: - (i) a small granular propagatory cell and (ii) a large somatic or ecto-dermal cell. Repeated division of the somatic cell further from ectoderm of larva.

Larval stage of Fasciola hepatica: -

Miracidium larva: -

It is the first larval stage in the life cycle of Fasciola hepatica.
It swims actively in water in search of secondary host, the snail of genus, Limnaea.
Miracidium larva does not feed. It swims actively in search of its secondary host.
In case it fails to reach the host, it dies within 24 hours.

Sporocyst larva: -

Miracidium lose its apical gland, penetrated gland, brain, eyespot and change into a sac-like sporocyst larva.
It looks like an elongated sac about 0.7mm long.

Redia larva: -

It is an elongated and cylindrical sac.
It comes out by the rapture of sporocyst.

Cercaria larva: -

A fully formed cercaria larva possesses a flattened heart-shaped body with a long contractile tail.
Its body surface covered with thin cuticle with backwardly directed spines.
The anterior end bears mouth, muscular pharynx, esophagus and bifid intestine.
The mouth is surrounded by oral sucker.

Metacercaria: -

The metacercaria is somewhat rounded with thick outer covering of cuticle in the from of cyst.
The Cystogrenous cells of the cercaria disappear and the flame cells increase in number.

Infection to the final host: -

The metacercaria enters the final host, the sheep, when it grazes on the aquatic weeds and reaches the intestine.

Infection: -

The vertebrate host (sheep, goat, etc.) gets the infection by grazing on grass, leaves and other vegetation to which metacercaria cysts are attached.
The invertebrate host (snail) acquires infection when a miracidium, at random, established contact with a suitable part of its body.

Pathogenesis or Symptoms: -

Infection of invertebrate host (snail) result in a partial or complete destruction of the affected site, which is preferably the digestive gland (liver) or gonad.
In case of heavy infection, snail considerably increases in size.
Significant economic important is the effect of F. hepatica on its vertebrate, host whose bile ducts as well as liver may be damaged.

Treatment: -

Anti-Helminthes drugs such as hexachloroethane, carbon tetrachloride, filcin, emetine hydrochloride, phenothiazine and tetrachloroethane are being employed or treating case of liver-rot.

Prophylaxis or Prevention: -

Killing heavily infected sheep.
Destroying eggs and manure of infected sheep.
Feeding infected sheep with salt and little dry food.
Killing or checking snail population.

Parasitic Adaptation of Fasciola: -

Liver fluke has undergone great modification, morphologically as well as physiologically, to suit its existence as an end parasite in the bile ducts of sheep.
Locomotory organs are absent as not required by adult. However, free swimming larvae, such as miracidium have cilia and cercaria has a locomotory tail.
Adult lacks circulatory, respiratory and sensory organs as they are not needed.
Nervous system is poorly developed for the same reason. However, free swimming miracidia has sensory eye sport.
Reproductive system is highly developed.
Number of eggs produced is enormous (about 200,000 eggs per fluke).
Hermaphroditism ensures self-fertilization even in the absence of another for copulation. It is necessary of the species.

Objective question of Platyhelminthes: -

1) Liver flukes belong to class: -

(a)Cestoda (b) Nematoda (c) Trematoda (d) Turbellaria

2) Fasciola hepatica is found in: -

(a)Intestine of sheep (b) Stomach of Sheep (c) Liver of sheep (d) None of them

3)Fasciola hepatica is: -

(a)Free living (b) Ectoparasite (c) Endoparasite (d) Ecto and Endoparasite

4) Which of the following infects the snail in life cycle of Fasciola?

(a)Cercaria (b) Miracidium (c)Redia (d) Metacercaria

5) Which of the following stage in the life cycle of Fasciola is infective stage for sheep?

(a)Cercaria (b) Miracidium (c)Redia (d) Metacercaria

6) The intermediate host in the case of liver fluke of is: -

(a) Snail (b) Man (c)Pig (d) fly

7) The correct sequence of various larva in liver fluke is: -

(a)Miradium ,sporocyst, redia ,cercaria, metacercaria

(b)Miracidium, sporocyst, cercaria, metacercaria, redia

(c)Cercaria, sporocyst, redia, metacercaria, miracidium

(d)Sporocyst ,redia , metacercaria, miracidium

8)Larva of Fasciola hepatica that bores into snail is: -

(a)Cercaria (b) Hexacanth (c) miracidium (d) Redia

9)The aperture presents at posterior end of Fasciola hepatica: -

(a) Anus (b) Excretory pore (c)Genital aperture (d) Cloacal aperture

10)Laurer's canal is present is: -

(a)Fasciola (b) Taenia (c) Ascaris (d) Cockroach

11)During its life cycle, Fasciola hepatica infects its intermediate host and primary host at the following larval stages respectively: -

(a)Redia and miracidium

(b) Metacercaria and cercaria

(c)Cercaria and redia

(d) Miracidium and metacercaria

12)In Fasciola, miracidium develops into the next stage inside: -

(a)Bulinus (b) Limnaea truncatula (c)Pila globosa (d) Planorobis

13) Planorbis and Limnaea are the intermediate of Fasciola?

(a)Faciola (b) Schistosoma (c) Trichinella (d) Echinococcus

14)Which of the infective stage to the primary host of Fasciola?

(a)Miracidium (b) Sporocyst (c) Metacercaria (d)Cercaria

15)Whichof the following larval stage of Fasciola does not produce several larva?

(a)Sporocyst (b) Secondary redial (c) Primary Redial (d)Cercaria

16)Which layer of Dugesia contains rhabditids?

(a) Epidermis (b) Circular muscle (c)Parenchyma (d)Cuticle

17) Fasciola hepatica (b) Taenia solium (c) Ascaris lumbricoides (d)Planaria

18) In Fasciola, the region where the shell gland opens into is the: -

(a)Oviduct (b) Ovovitelline duct (c) Uterus (d) Oocyte

19) Fasciola hepatica is a parasite that lives in the: -

(a)Intestine of sheep (b) Liver of sheep (c)Spleen of sheep (d)Pancreas of sheep

20)In Fasciola ,the germ cells of redia gives rise to: -

(a)Daughter Cercaria (b) Daughter Sporocyst (c) Daughter Redia (d)Daughter Metacercaria

21) Liver fluke is not affected by enzymes of host because of: -

(a)Cuticle (b) Suckers (c) Sporocyst (d) Cysticercus

22)The stage of life history the liver fluke when it infects the primary host is: -

(a)Redia (b)Cercaria (c) Sporocyst (d) Cysticercus

23)Which stage of Liver fluke infects the intermediate host?

(a)Redia (b)Cercaria (c) Sporocyst (d) Cysticercus

24)Primary host of Fasciola is: -

(a)Man (b) Pig (c) Sheep (d) Dog

25)For Attachment, fasciola has: -

(a)Four suckers and hook (b) Two suckers (c) Two suckers and hooks (d) Hooks only

26)Correct sequence of larval stages during development of fasciola is: -

(a)Miracidium, Redia, Sporocyst, and Metacercaria

(b)cercaria, Miracidium, Sporocyst and Metacercaria

(c)Miracidium, Sporocyst, Redia, Cercaria and Metacercaria

(d)Miracidium, Sporocyst, Redia, Cercaria and Metacercaria

27)In Fasciola, germ ball of Redia give rise to: -

(a)Daughter sporocyst (b) Daughter Cercaria (c) Daughter Redia (d) Metacercaria

28)Which is the first stage of larva of Fasciola?

(a)Miracidium (b) Sporocyst (c)Redia (d)Cercaria

29)Fasciola has: -

(a)Salivary gland (b)Pharyngeal gland (c) Gastric Gland (d) Intestinal gland

30)The cause of 'liver rot ' in sheep: -

(a) Dugesia (b) Fasciola (c) Taenia (d) Schistosoma

👉 Answer Key: -

1.(c)2.(c)3.(c)4.(b)5.(d)6.(a)7.(a)8.(c)9.(b)10.(a)11.(d)12.(b)13.(a)14.(d)15.(d)16.(a)17.(a)18.(b)19.(b)20.(c)21.(a)22.(b)23.(b)24.(b)25.(b)26.(c)27.(c)28.(a)29.(b)30.(b)

👇Terminal Question 👇

1.Give an account of the life cycle of Fasciola hepatica.

2.What are adaptations? Explain with reference to the structure of Fasciola hepatica.

3.Where is Fasciola hepatica occurrence? Give an account of its structure.

4.Describe reproduction organ of Fasciola hepatica? (Only Diagram).

5.Describe the various larval form of Fasciola hepatica.

6.What is the infective stage of Fasciola hepatica for sheep?

7.Which organism is the secondary host of Fasciola?

8.What is miracidium?

9.To which larva does miracidium larva give rise to?

10.What are flame cells?

11.Where can you located flame cells?

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Friday, February 24, 2023