12 Representative Types of Platyhelminthes

The following points highlight the twelve representative types of Platyhelminthes. The types are: 1. Convoluta 2. Thysanozoon 3. Gyrodactylus 4. Aspidogaster 5. Opisthorchis 6. Schistosoma 7. Gyrocotyle 8. Tetrarhynchus 9. Diphyllobothrium  10. Taenia Saginata 11. Echinococcus 12. Hymenolepis.

Platyhelminthes: Type # 1. Convoluta:

Convoluta (Fig. 43.1) is exclusively marine form living under stones among algae. It is a small worm with sides of body curved ventrally. Anterior end has a cluster of frontal glands, a pair of eyes and a statocyst. Mouth situated ventrally near the anterior end. Intestine is absent.

Excretory system is completely absent. Hermaphroditic with protandrous condition. Convoluta exhibits symbiotic phenomena by having algal cells in symbiotic association in its body.

Platyhelminthes: Type # 2. Thysanozoon:

Thysanozoon (Fig. 43.2) is found commonly in a less cold water of sea.

Body is covered with numerous papillae each containing an intestinal branch. Anterior end bears a pair of marginal tentacles and numerous cerebral eyes. Pharynx is tubular. Glandulomuscular adhesive organ is present on the dorsal surface behind the female gonopore. Hermaphroditic with a pair of male pore and single female pore. Seminal bursa is absent.

Platyhelminthes: Type # 3. Gyrodactylus:

Gyrodactylus (Fig. 43.3) 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 sac-like 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.

Platyhelminthes: Type # 4. Aspidogaster:

Aspidogaster (Fig. 43.4) 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 an oral 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 simple without alternation of host.

Platyhelminthes: Type # 5. Opisthorchis:

Opisthorchis sinensis (Fig. 43.5) (formerly called Clanorchis). It is called a Chinese liver fluke. It is parasitic in the bile ducts of man in China, India, Japan and Indo-China, it is also found in fish-eating mammals. The size ranges from 10 to 25 mm.

The two suckers are small, the alimentary canal is well-formed, excretory bladder is Y-shaped. Male organs—Two branched testes lie one behind the other in the posterior region, a vas efferens arises from each testis, the two vasa efferentia unite in the middle of the body to form a vas deferens which joins a seminal vesicle.

A narrow ejaculatory duct arises from the seminal vesicle to open into a genital atrium which opens by a gonopore on the ventral surface just in front of the acetabulum. A penis, prostate glands and cirrus are lacking. Female organs—A small lobed ovary lies in front of the testes. From the ovary arises a short oviduct.

In the middle one-third of the body on each side are small follicles of vitelline glands. Small ducts arise from vitelline glands to form two transverse vitelline ducts which unite to form a small common vitelline duct.

The common vitelline duct joins the oviduct, after which the oviduct joins an ootype which is surrounded by small cells of Mehils’s gland. In the ootype, the egg and yolk get enclosed in a shell to form a capsule which has an operculum and comma-shaped appendage.

From the ootype arises a long, coiled uterus containing capsules, it opens into the genital atrium. Behind the ovary lies a sac-like seminal receptacle from which a small duct joins the oviduct, but before it joins the oviduct, it receives a Laurer’s canal. The Laurer’s canal curves behind the seminal receptacle and opens dorsally by a small pore in the middle.

Fertilised eggs pass out of the gonopore into bile ducts from where they reach the human intestine and go out with the faeces. The capsules do not hatch unless eaten by snails Parafossalurus or Bythinia. In the snail’s intestine, miracidium larvae hatch from capsules, they bore into tissues of the snail.

The miracidium gives rise to a rounded sporocyst. Sporocyst produces rediae which have no birth pore. The rediae produce cercaria larvae. A cercaria has a long tail with fluted lateral fins.

The cercariae escape from the snail to enter a second intermediate host which is a fish belonging to the carp or minnow family. The cercariae encyst in the muscles of the fish. When raw or insufficiently-cooked fish is eaten, young flukes emerge from the cysts in the human small intestine from where they reach the bile ducts within a few hours; in three weeks the flukes mature.

Platyhelminthes: Type # 6. Schistosoma:

Schistosoma (Fig. 43.6) (old name Bilharzia). It is a dioecious digenetic trematode, being parasitic in the human hepatic portal or pelvic veins; some species are parasitic in veins of birds and mammals.

It is peculiar in having separate males and females, but the two are found together in pairs. A thick male permanently carries a slender but longer female in a gynaecophoric canal formed by folding of the ventral body wall. Body surface is rough and spiny, and both have an oral sucker and an acetabulum.

There is no pharynx, and the two branches of the intestine reunite in the middle of the body, this character and being dioecious distinguish these blood flukes from other trematodes.

Male organs—The male has four testes; a short vas deferens arises from the testes and joins a seminal vesicle which enters a penis, the penis opens by a gonopore below the acetabulum. Female organs—The female has an elongate ovary above the point where the intestinal caeca re-join, from the ovary an oviduct passes in front.

In the posterior third of the female are vitelline glands from which a vitelline duct joins the oviduct. The oviduct meets an ootype surrounded by Mehlis’s gland. From the ootype arises a straight uterus containing a few capsules, it opens by a female gonopore below the acetabulum. After fertilisation, the female leaves the male to lay eggs one at a time in the smaller blood vessel.

The capsules lacerate the capillaries and reach the urinary bladder and are Voided with urine. Immediately the urine is diluted with water, the capsules hatch into miracidium larvae which enter a freshwater snail and reach its liver. The miracidium forms a sporocyst. The first generation of sporocyst may again produce miracidium larvae, which then form a second generation of sporocysts.

Either the first generation or the second generation of sporocysts can give rise to cercaria larvae which have forked tails. There is no redia stage. The cercariae come out of the snails and swim freely in water, and without encysting they penetrate the human skin with great rapidity during bathing or washing or they may be swallowed by drinking infected water.

Cercariae enter the blood vessels, go to the heart, then lungs, and then to the liver where they grow; after that they enter the hepatic portal or pelvic veins and become sexually mature. The cercariae developing from one egg will produce flukes of only one sex, and females do not mature in a host where no males are present. When a male finds a female, it encloses her in the gynaecophoric canal.

There are three species of Schistosoma parasitic in human beings. 1. S. haematobium is found in Africa, Palestine, Iraq and Portugal in human pelvic veins. Its capsule has a sharp terminal spine, its intermediate host is Bulinus. 2. S. mansoni is found in Africa and tropical America in the veins near the ileocaecal junction.

Its capsule has a lateral spine, the intermediate host is Planorbis. 3. S. japonicum is found in Japan; China and Philippines in the hepatic portal and mesenteric veins. It is also parasitic in dogs, cats, cattle, horses and pigs, its capsule is small and has a rudimentary lateral spine, its intermediate host is Oncomelania.

S. indicum is found in portal veins of Indian cattle. Schistosoma causes bladder injuries, bladder stones, skin disease and haematuria which is a disorder of kidneys with discharge of blood. In Egypt, 60% of the population is infected. The diseases can be prevented by sanitary control of water, and they can be cured by compounds of antimony.

Platyhelminthes: Type # 7. Gyrocotyle:

Gyrocotyle (Fig. 43.7) 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.

Platyhelminthes: Type # 8. Tetrarhynchus:

Tetrarhynchus (Fig. 43.8) 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 a eversible proboscides armed with spines. Proboscides or rostella are enclosed in muscular sheaths which end below in muscular bulbs. Reproductive system single in each proglottid.

Platyhelminthes: Type # 9. Diphyllobothrium:

Diphyllobothrium latum, (Fig. 43.9) the broad fish tapeworm, is the largest and most injurious parasite in the intestine of man. Body length reaches up to 18 metres and have 3,000 to 4,000 proglottides. Scolex is fusiform, bears two slit-like bothria, followed by the long slender neck.

Mature proglottid is broader than long. Excretory system 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. Diphyllobothrium latum is world-wide in distribution.

Platyhelminthes: Type # 10. Taenia Saginata:

Taenia saginata (Fig. 43.10) commonly called beef tapeworm is found in intestine of man. It is cosmopolitan in distribution. The body of the worm is dorsoventrally flattened and much larger than Taenia solium, reaches a length of 5 to 6 metres.

Body is divisible into scolex, neck and strobila. Scolex is 1.5 to 2 mm in diameter, bears four large suckers for adhesion and devoid of rostellar hooks. Both mature and ripe proglottids are larger than those of T. solium. Life history is similar to that of T. solium. Intermediate hosts are cattle, usually cow and buffalo. Infection takes by ingestion of improperly cooked beef.

Platyhelminthes: Type # 11. Echinococcus:

Echinococcus granulosus (Fig. 43.11) is a minute tapeworm, 3 to 6 mm long. It is common in Africa, Australia, New Zealand, Iceland, Holland and South America.

It is found in hundreds in the intestine of dogs, cats and wolves. The scolex has four suckers and a protrusible rostellum with two rows of hooks, there is a neck and 3 to 4 proglottides, one immature, one or two mature with complete hermaphrodite reproductive organs, and one large gravid proglottis.

The eggs pass out with the faeces 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 embryo bores its way generally 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 fibrous cyst wall. It is now called a hydatid cyst. (Fig. 43.12). Development of the cyst is slow and after months or even years the bladder wall produces, not scolices, but hollow brood capsules which remain attached by slender stalks or fall free into the fluid-filled cavity of the cyst.

With age more brood capsules are formed and older brood capsules form from 3 to 30 scolices on their inner walls. Sometimes due to pressure hernia-like buds arise from the mother cyst, these are daughter bladders or cysts. The daughter cysts have a fibrous cyst wall and a bladder wall, they may arise inside the mother cyst by endogenous budding or externally by exogenous budding.

The exogenous daughter cysts may detach themselves and migrate, they continue their development in some other part of the body. The daughter cysts also give rise to several scolices from their inner bladder wall.

The hydatid cyst has a colourless fluid which may be from 2 to 50 quarts, but older cysts have a granular deposit consisting of brood capsules and free scolices. The scolices are finally evaginated in brood capsules and in endogenous and exogenous daughter cysts, and should then reach the final host, a dog, cat or wolf, they develop into adult Echinococcus.

Hydatid cysts are often enormous and the liver is enlarged by them, in the brain or eye they prove fatal. The hydatid fluid contains toxins and should it leak from the cyst, eosinophilia results. If the hydatid cyst is ruptured by pressure, not only is the toxic fluid liberated, but the scolices, brood capsules and daughter cysts become scattered in the body and each may develop into a new cyst.

Platyhelminthes: Type # 12. Hymenolepis:

Hymenolepis nana (Fig. 43.13) 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. Hymenolepis nana causes severe toxic symptoms including abdominal pain and diarrhoea, etc. It is worldwide in distribution. It is the commonest tapeworm in Southern United States.