CLASS TREMATODA: Flukes (trematodes, perforated form)
This class has eight thousand species of parasitic flatworms. Trematoda are collectively called flukes. They are wide and flat shaped.
1. Habitat: Almost all adult flukes are parasites of vertebrates. Their immature stages are found in vertebrates or invertebrates host. They are also encysted on plants. Many species are of great economic and medical importance.
2. Size: Most flukes are flat and oval to elongate. Their size ranges from less than 1 mm to 6 cm in length.
3. Nutrition: They feed on host cells and cell fragments. The digestive tract includes a mouth and a muscular, pumping pharynx. The digestive atrium tract divides into two blind-endings and branched pouches. These pouches are called cecae (sing., cecum). Some flukes also absorb nutrients across their body wall.
Body wall: Their body wall shows evolutionary adaptation for the parasitic way of life. The epidermis consists of an outer layer called the tegument. The tegument forms a syncytium. Syncytium is a continuous layer of fusedcells. The outer zone of the tegument is called glycocalyx. It consists of an organic layer of proteins and carbohydrates.
Fig: Generalized fluke (Digenetic Tremaatode)
The glycocalyx helps in the transport of nutrients, wastes, and gases across me body wall. It also protects the fluke against enzymes and the host’s immune system. Microvilli also found in this zone. These help in nutrient exchange. Cytoplasmicbodies contain nuclei and most pf the organelles. These lie below the basement membrane. Some slender cell possess called cytoplasmic bridge are also present. These processes connect the cytoplasmic bodies with the outer zone of tegument.
Class Aspidogastrea consists of a small group of flukes. They are primarily internal parasites (endoparasites) of molluscs. They have a large, oval holdfast organ called the opisthaptor. It covers the entire ventral surface of the animal. It is present in all aspidogastreans.
The cristhaptor is an external long attachment organ. It is subdivided by ridges or septa. The oral sucker is absent. The life ycle of aspidogastreans involves only one host (a mollusc) or two hosts. In case of two hosts. the final host is a vertebrate me (fishes or turtles). It is infected by ingesting a mollusc.
Majority of a flukes belong to this subclass. In this subclass, two different forms develop. One form is adult and other is one or more larval stages. The digenetic flukes complete their life cycle in at least two different hosts. These animals possess the most complex cycle in the entire animal kingdom.
The a tilt forms of digenean are endoparasites in the blood streams, digestive tracts, ducts of the digestive organs, or other visceral organs. Their definitive or final hosts are different types of vertebrates. They have one or more intermediate hosts. Immature stages in the form of different larval stage live in these hosts. The adhesive organs are two large suckers. The anterior sucker is the oral sucker. It surrounds the mouth. Another sucker called acetabulum is located ventrally on the middle portion of the body.
Life cycle of Digenea .
1. Egg: The eggs of digenetic trematodes are oval. These have a lid like hatch called an o rculum.
2. Miracidium (pl; miracidia): The egg reaches freshwater. Its operculum opens. A ciliated larva called a miracidium swims out. It finds a suitable first intermediate host (a Snail).
3. Sporocyst: The miracidium penetrates the snail. It loses its cilia and develops into a sp u–ocyst. Sporocysts are bag like structures.
4. Daughter sporocysts or rediae (sing; redia): Miracidium contains embryonic cells. These cells develop into daughter sporocysts or rediae by asexual reproduction. Hundreds of rediae are formed from a single miracidium.
5. Cercariae (sing; carcaria): Embryonic cells are also present in each daughter sporocyst or rediae. They produce hundreds of the next larval stage cercariae. The phenomenon of producing many cercariae is called polyembryony. It increases the chance of finding a host. Cercaria has a digestive tract. suckers, and a tail.
6. Metacerearia (pl; metacercariae): Cercariae come out of snail. They swim freely and find a second intermediate or final host. This host may be a vertebrate. invertebrate, or plant. Cercaria penetrates into this host. It forms cyst and becomes metacercaria. The definitive host eats the second intermediate host. The metacercaria remove cyst and develops into an adult.
Some Important Trematode Parasites of Humans
Chinese liver fluke, Clonorchis sinensis
It is a common parasite of humans in Asia. Over 30 million people are infected by it. Its adult form lives in the bile ducts of the liver. It feeds on epithelial tissue and blood. The adults release embryonated eggs into the common bile duct. The eggs enter into the intestine and are eliminated with feces. Snail ingests the eggs. Miracidia are released From egg. Then sporocyst and redial stages, cercariae emerge into the water. Cercariae contact a fish (the second. intermediate host). It penetrates the epidermis of the fish. It loses its tail and form cyst. Some human eats raw or poorly cooked fish. The met icerearia enter into their digestive tract. It develops into an adult.
Fasciola hepatica, Sheep liver fluke
It i common in sheep-raising areas. It uses sheep or humans as its definitive host. The adults live in the bile duct of the liver. Eggs pass passes through the common bile duct to the intestine. They are eliminated through anus. Eggs are deposited in freshwater.
They hatch and release miracidia. Miracidia locate the proper species of snail. They penetrate the snail’s soft tissue and develops into sporocysts. Sporocysts give rise to rediae. Rediae give rise to cercariae. The cercariae emerge from the snail. They forth cyst on aquatic vegetation. Sheep or other animals become infected during grazing. Humans may become infected with it by eating a freshwater called watercress. These plants contain encysted metacercaria.
Schistosomes, blood flukes
Schistosomes have great medical importance. They infect over 200 million people throughout the World. Infections are most common in Africa, South and Central and Southeast Asia. The adult is dioecious worms. They live in the human bloodstream. The male fluke is shorter and thicker than the female. The sides of the male body curve under. It terms a canal along the ventral surface (schistosoma means split body). The female fluke is long and slender. It is carried in the canal of the male. Copulation is continuous and the female produces thousands of eggs over her lifetime. Each egg contains a spine. This spine helps in the movement of it through host tissue. These eggs are eliminated in feaces or urine. The eggs of schistosomes lack operculum. miracidium escapes through a slit into water. The miracidium finds a snail by chcmotaxis. The miracidium penetrates it. It develops into a sporocyst and then daughter sporocysts. It finally forms cercariae. The cercariae have forked tail. There is no rediae generation in it. The cercariae leave the snail. It penetrates the skin-of a human. It has anterior glands. They glands secrete digestive enzyme that help in penetration. The cercariae lose their tails in human. They develop into adults in the intestinal veins. Metaccrcaria stage is absent in them.