File: <platyhelminthes

The Phylum: Platyhelminthes includes the flatworms (Planaria, flukes & tapeworms) Three Classes treated here are Turbellaria, Trematoda and Cestoda.

The Class Turbellaria is distinguished mainly on the appearance of the digestive tract. Four orders discussed are Acoela, Rhabdocoela, Tricladida and Polycladida as follows:

Order Acoela (= "no gut"). There is a mouth and pharynx but no digestive cavity and so are similar to the planula larva. Endoderm cells distribute food.

Order Rhabdocoela (= "straight gut"). These have a straight digestive cavity. Both fresh water and marine forms occur. Asexual reproduction involves the production of chains of individuals, which remain attached to each other. Nematocysts are aacquired from a Cnidarian diet.

Order Tricladida (= 3-branches"). There are three main branches of the digestive tract, such as is found in the genus Planaria. There are mostly fresh water forms.

Order Polycladida (= "many branches"). All are marine and branches of gut run out radially from the pharynx.

Simple sketches of the four orders are shown in the following diagram:

Platyhelminthes includes the familiar Planaria of elementary classrooms and the appalling flukes and tapeworms that have plagued humans for millennia. The phylum is distinguished by showing bilateral symmetry, allowing for such designations as anterior, posterior, dorsal, and ventral. The animals are dorso-ventrally flattened, and they possess three distinct germ layers during development and as adults. A distinct mesoderm layer is first presented here. The digestive tract has only one opening, which is similar to that of Cnidaria but more organized. This also serves as the anus. Flame cells are present, which are excretory cells that are the first structures to appear, which are exclusively excretory in function.

The flatworms are found widely distributed in both fresh and salt waters, soil and they occur as parasites of other animals. They have a well developed system of organs and their reproductive system is especially well developed.

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The Class Turbellaria is named such by the little turbulences they create by the beating of their epidermal cilia. These are mostly free-living organisms and only a few are commensals and parasites. The body surface, or epidermis, is covered with cilia. They are found in fresh and salt water and in the soil, and their size ranges up to about 5 cm. [See Inv159 for example]

A representative Genus is Dugesia (= Planaria). Their habitat is under objects in fresh water and so they are readily available over a wide geographic range in North America.

Body Plan.-- An elongated, ribbon-like animal exceedingly flat. A proboscis is carried in a sheath when not in use. There are two eyespots with a pigment that suggests a "wall eye." An auricle is present (see diagram).

Body Wall. There are cilia on the epidermis, and the epidermis contains rhabdites, which may function to secrete mucous material, which covers the body. Rhabdites are produced in sub epidermal cells and are later moved to the epidermal cells. Circular muscles form a complete layer around the body circumference. Longitudinal muscles are formed around the circular muscles. A Parenchyma (mesenchyme) is a disorganized tissue, which fills up the entire interior of the body that is not occupied by other organs. Therefore, there is no other cavity than the digestive one.

Food & Digestion.-- The species are carnivorous and will catch and feed on anything around. The mucous on the body of Dugesia entangles the prey, while the proboscis enters the prey to suck its contents. The pharnyx empties into the digestive cavity, which in turn consists of three parts leading to all sections of the body. Digestion commences extracellularly but is terminated intracellularly, just as in the Cnidaria. There is no anus.

Circulation.-- Food is circulated before it is digested. Distribution is accomplished by diffusion, and flagellated gastrodermal cells keep the food in motion.

Respiration.-- Simple diffusion accomplishes respiration that is similar to previous phyla discussed. This is an advantage of having a flat body.

Excretion.-- Flame cells accomplish excretion. A hollow cavity occurs inside the flame cell and a tuft of cilia lies in the cavity. Wastes enter the cell and diffuse through the cavity and then are forced out into the excretory tube by the beating of cilia. Excretory tubes lead to a common channel, which has two body openings.

Support.-- Little support is required for planarians living in fresh water.

Protection.-- The mucous, which is a very sticky mass, entangles would-be predators.

Motion.-- There are two kinds of motion in Dugesia. The smooth, gliding motion is achieved in part by the beating of cilia and in part by muscular contractions of the body wall.

Sensitivity.-- Three structures are sensitive. Eye spots on the animal are a shade that overhangs light-sensitive cells. Auricles are sensitive to chemical stimulation and aids in the location of food. The whole body surface also is sensitive to touch and chemical stimuli.

Nervous System.-- A cerebral ganglion is present and there are two nerve cords on the ventral side that run the length of the body from the anterior cerebral ganglion. "Ladder type" commeasures connect the nerve cords. Various nerve processes extend off of the nerve cords. This is the first nervous system with a coordinating ganglion and the first to show nerve cords.

Reproduction.-- Both sexual and asexual reproduction is found in Dugesia. The sexual reproduction is the best developed of all systems. The animals become sexually mature only at rare intervals, which is triggered by conditions in the environment. They are hermaphroditic but cross-fertile. Any one animal can act as male or female, but only one sex at a time.

The male sexual structures consist of testes, vasa efferentia (2), vasa deferentia (2), a penis and a genital pore. The female sexual structures consist of ovaries, an oviduct, a vagina, a uterus, a bursa and a genital pore. The genital pore is common to both male and female.

Eggs are laid in a cocoon and vitellaria, or yolk glands, are present. These glands secrete yolk in the form of actual cells. The yolk cells are collected down in the vitellaria and serve to provide food for the developing embryo. The eggs are ectolecithal as the yolk occurs on the outside of the egg.

During asexual reproduction the posterior part of the animal simply breaks off. This is the most common means of reproduction in Dugesia.

Regeneration.-- This has been widely studied in this group of animals. There is a polarity that exists in each cut piece. The edge that was closest to the anterior end of the animal will develop a new head, and that closest to the posterior end will develop a new tail. Sections cut from the center of the animal will develop into a whole new animal.

Economic Importance.-- No direct importance, but planarians have been used to combat mosquitoes with some success (see Mosquito Suppression with Planaria)

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Review of The Turbellaria

There are many variations in the form of the digestive tract and the position of the mouth. Many species possess suckers. Sexual reproduction is found in all, but asexual reproduction is the most common in the group. In Rhabdocoela there are nematocysts, which are acquired from consuming Cnidiarians. Most Turbellaria have direct development. Eggs are held in a cocoon from which adults hatch. A free-swimming larval stage does occur, which is known as Muller's Larva.

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The Class Trematoda include the flukes, and two Subclasses will be presented here: Monogenea and Digenea. Monogenea is a small group whose members are external parasites on the gills of fish, in the mouth cavity, etc., and some are parasites of cold-blooded vertebrates and some invertebrates. There is a direct development without a larval stage or intermediate host. Digenea are internal parasites, mostly on vertebrates. There is an indirect development where larval stages are living in intermediate hosts. The life cycles of the Digenea are quite complex. [See Inv160 for example]

All species of Trematoda are parasitic but they are mostly internal parasites, with fewer numbers being external parasites. They are generally similar in structure to the Turbellaria but with several modifications. They have more complicated life cycles that involve several host animals.

A representative Genus is Pneumonoeces, which includes the lung flukes of frogs and toads. The adults of these animals live in the lungs of frogs and toads, while the larvae are parasitic on snails and dragonflies.

Body Plan.-- This is similar to Dugesia, but the mouth is located anteriorly and is equipped with a sucker. There is also a sucker on the ventral surface called the acetabulum that has no opening.

Body Wall.-- An epidermis is nonexistent in adult flukes. They have very poorly developed muscle layers. The mesenchyme bears cells, which secrete the cuticle. The cuticle protects the fluke from the host's digestive fluids. There are no cilia, hence no locomotion!

Food & Digestion.-- Food is derived from the cells, mucous, blood, etc. of the host's lung. A muscular pharynx located behind the oral sucker serves as a food pump. There is a single-branched intestinal tract that has no subdivisions. An exception is in the sheep liver fluke.

Circulation, Respiration & Excretion.-- These functions are the same as in the Turbellaria.

Support & Protection.-- The heavy cuticle supports and protects the animal.

Locomotion. -- There is very little locomotion as there is not much occasion for movement. Muscle layers are weakly developed.

Sensitivity.-- Although present in the larvae, there are no sense organs in the adult. Sensory cells are generally diminished as compared to the Turbellaria as there are little external stimuli that reach these animals due to their interior environment.

Nervous System.-- The nervous system is of the same pattern as in Dugesia, but it is not as well developed and much reduced.

Reproduction.-- Both sexual and asexual reproduction is present. In sexual reproduction the animal is hermaphroditic and is self or cross-fertile. Males have two testes and a cirrus, which turns inside out as it is protruded. It is the expanded, eversible end of the vas deferens. The base of the cirrus stores sperm and is the seminal vesicle.

The female has one ovary, and the uterus winds around and empties out at the top of the cirrus sac. A shell plus about 12 yolk cells are put around the eggs. The eggs are then laid in tremendous numbers and already contain embryos at the laying time.

Asexual reproduction is found in the life cycle in different hosts. There are such structures as germ bulbs, rediae, etc.

Life Cycle. -- In Pneumonoeces the frog coughs up the eggs, they are swallowed and passed out with the faeces. The miracidium larval stage then commences. It is covered with cilia and possesses a pair of eyes and an epidermis. It swims around in the water until it finds a certain kind of snail. It then bores through the snail body and turns into a sporocyst.

The sporocyst develops germ bulbs within its body, each bulb developing into the next larval stage, the redia. The redia taken on more characteristics of the fluke. When the redia is mature it bursts open to release more rediae. Inside of these rediae are formed the third larval stage called the cercaria.

The cercaria has a tail, but otherwise it resembles a fluke. In this stage the snail body is departed and the cercaria swims around by virtue of its tail. There are eyes present here. Dragonfly nymphs may consume the cercaria or they may simply bore through the dragonfly nymph's body wall. The gut of the dragonfly is penetrated and a cyst is formed called a metacercaria.

The metacercaria remains in the body of the nymph for its entire development, which may take two or three years. When a frog consumes a dragonfly it obtains these metacercaria. The metacercaria then burrow into the frog's gut, enter the blood, reach the lungs and change into adult flukes.

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Modifications For Parasitism in Trematoda .--There is a loss of epidermis and the acquisition of a heavy cuticle. The presence of suckers enables the animals to hang on to various internal or external locations. There is a general reduction or weakening in the muscular system, and a loss of sense organs and reduction in size and complexity of the nervous system as a whole. The digestive tract is reduced and less branched, which is correlated with the ease that food may be digested. A tremendous increase in the development of the reproductive system occurs, which enables the production of very large numbers of eggs. These animals are also able to reproduce asexually in the larval stages.

Probable Evolution of Complex Life Cycles.-- The Trematoda were probably the original parasites of snails and the snail usually is the first intermediate host. With the passage of time and consumption of snails by other animals, particularly arthropods, it is naturally assumed that Arthropoda were the next in line. In a like manner vertebrates feeding on Arthropods acquired the parasites. The Class shows much variation in morphology and life cycles such as the addition of and the deletion of steps.

Economic Importance of Trematoda.-- Almost every species of Vertebrate has one Trematode parasite. Intestinal flukes are the most common so that the intestine was probably the original site. Clonorchia is a liver fluke of humans causing "Oriental Fluke" disease. There are also lung flukes and blood flukes.

Schistosoma is a human blood fluke of the tropics. It irritates and clogs blood vessels. There is an active migration of eggs via the urinary system to the outside, causing a serious and painful aspect of this disease. Skin irritation caused by cercaria boring through the skin causes "Swimmers Itch." However, in this case the cercaria die. There are separate male and female sexes, with the male possessing a grove in his body where the female resides.

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The Class Cestoda includes the tapeworms of humans and animals. All species are internal parasites primarily of vertebrates. Their head is modified into a scolex, which is principally an organ of attachment. Their neck is located behind the scolex. Behind the neck are the proglottids that are segments, which increase in size posteriorly. They are budded off in the neck region and mature as they move backwards. The Cestoda have lost all traces of a digestive tract. [See Inv161 for example]

A representative species is the pork tapeworm, Taenia solium. As an adult it lives in the human intestines while the larvae are encysted in the muscles of hogs. It has a simple morphology consisting of a scolex or rostellum adapted with hooks and four suckers, a neck and 800-1000 proglottids per animal.

Body Wall.-- There is no epidermis but rather a heavy cuticle. Muscle layers lie beneath the cuticle. A transverse muscle layer runs through the parenchyma and is not circular.

Food & Digestion.-- There is no gut but rather the whole body surface is involved in the absorption of food, which is predigested by the host.

Circulation.-- There is a simple diffusion throughout the body.

Respiration.-- There is generally low oxygen content in the host gut and therefore anerobic respiration is common.

Excretory System.-- Flame cells and excretory canals are present. There are two longitudinal canals with a transverse connection in each proglottid.

Support & Protection.-- The hooks and rostellum of the scolex provides support. The body wall affords protection. Although its cuticle is digested, it continues to be reformed at a rate equal to that, which is digested. Amazingly enough it is still permeable to food.

Locomotion. -- There is little movement in the host gut. The proglottids may creep around after they break off of the main chain, however.

Sensitivity.-- There is no nervous system and no sense organs present.

Reproduction.-- Tapeworms are reproductive machines, Each proglottid has a complete set of male and female organs. In some cases there may even be two sets of each. All reproductive organs lie inside of the transverse muscles.

The male has testes scattered throughout the proglottid. The vasa efferentia run into a vas deferens. A large portion of the vas deferens is swollen and is called the seminal vesicle. A cirrus and cirrus sac lie at the side. There is a male genital pore that is not common with a female pore.

The female has a single bilobed ovary and a short oviduct. A Mahlis gland or "Shell gland" is present. There is a long vagina and the uterus has no opening to the outside but with its many branches serves for egg storage. A proglottid whose uterus is full of eggs is called a gravid proglottid.

Copulation.-- Copulation may occur between two proglottids although self-fertilization also exists.

Egg Development.-- The eggs are liberated by the gradual disintegration of the proglottid itself. The outer shell sloughs off and this leaves the embryo enclosed in a shell of its own, which is called the hexacanth embryo.

The hog picks up the hexacanths and these then penetrate its intestines, enter into the blood stream and become relocated in the muscles where they encyst. The encystment is called cysticercus. The cysticercus begins to develop an inverted scolex. After humans consume raw pork, the scolex everts in the human intestines and hooks itself in the intestinal wall from whence it proceeds to develop a tapeworm.

Importance.--The worm's mere bulk may cause clogging of the gut and the hooks may burrow deep enough to cause secondary infections.

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Another representative species, Taenia saginata is the beef tapeworm. There are no hooks or rostellum, and a cow is the intermediate host. This species is more prevalent wherever humans consume raw beef.

A third representative species is Diphyllobactrium latum, the fish tapeworm. Some individuals can reach up to 10 meters in length. Its life cycle is complex. The intermediate hosts are Cyclops and freshwater fish; the definitive hosts are humans. A ciliated embryo attacks the Cyclops in freshwater:

A third representative species is Echinococcus granulosis. The adult stage occurs in foxes, wolves and dogs. Intermediate hosts are humans, deer, etc. The animal proliferates inside the host and my reach all parts of the body. Humans are most likely accidental hosts.

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Please see following plates for Example Structures of the Platyhelminthes:

Plate 21 = Platyhelminthes: Turbellaria: Tricladida: Dugesia agilis (whole animal)

Plate 22 = Platyhelminthes: Turbellaria: Tricladida: Dugesia agilis (cross-section of Plate 21)

Plate 20 = Platyhelminthes: Turbellaria: Tricladida: Dugesia agilis

Plate 23 = Platyhelminthes: Turbellaria: Proctyla sp., Bdelloura sp., Leptoplana sp., Dugesia sp.

Plate 24 = Platyhelminthes: Trematoda: Prosthogonimus macrorchia

Plate 25 = Platyhelminthes: Trematoda: Digenea -- Example Structures & Life Cycle

Plate 26 = Platyhelminthes: Trematoda: Digenea: Clonorchia sinensis, Distomaria hepatica,

Paramphistomum sp., Octomacrum sp., Notocotylus sp. & Clonorchia sp.

Plate 27 = Platyhelminthes: Cestoda: Taenia pisiformis & Taenia serrata

Plate 28 = Platyhelminthes: Cestoda: Echonococcus granulosis, Taenia pisiformis, Dibothria canium

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Bibliography

Introduction Nemertinea►