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Helicometra, an opecoelid digenean

from the intestine of a fish

Scientific classification


Digenea (Gr. Dis - double, Genos - race) is a subclass within the Platyhelminthes consisting of parasitic flatworms with a syncytial tegument and, usually, two suckers, one ventral and one oral. They are particularly common in the digestive tract, but occur throughout the organ systems of all classes of vertebrates. Once thought to be related to the Monogenea, it is now recognised that they are closest to the Aspidogastrea and that the Monogenea are more closely allied with the Cestoda. Around 6000 species have been described to date.



Key Features

Characteristic features of the digenea include a tegument. They possess a vermiform, unsegmented body-plan.

There are typically 2 suckers, an anterior oral sucker surrounding the mouth, and a ventral sucker sometimes termed the acetabulum, on the ventral surface.

Monostome is a term used to describe worms with one sucker (oral). Flukes with an oral sucker and an acetabulum at the posterior end of the body are called Amphistomes. Distomes are flukes with an oral sucker and a ventral sucker, but the ventral sucker if somewhere other than posterior

Reproductive System

Adult digeneans are commonly hermaphroditic. This is likely to be an adaptation to rarity, allowing the life cycle to continue when only one individual successfully infects the final host. Fertilisation is internal, with sperm being transferred via the penis to the Laurer's Canal or genital aperture. A key group of digeneans which are dioecious are the schistosomes. Asexual reproduction is also possible in the first larval stage. While the formation of the digenean eggs and of asexual reproduction in the first larval stage (miracidium) is well reported, the developmental biology remains a problem which is very complex and far from being solved. Electron microscopic studies have shown that the light microscopically visible germ balls consist of mitotically dividing cells which give rise to embryos and to a line of new germ cells that become included in these embryonic stages. Since the absence of meiotic processes is not proven, the exact definition remains doubtful.

Male Organs

Protandry is the general rule among the Digenea. Usually 2 testes are present, but some flukes can have more than 100. Also present are vasa efferentia, a vas deferens, seminal vesicle, ejaculatory duct and a cirrus (analogous to a penis) enclosed is a cirrus sac

Female Organs

Usually there is a single ovary with an oviduct, a seminal receptacle, a pair of vitelline glands (involved in yolk and egg-shell production) with ducts, the ootype (a chamber where eggs are formed), a complex collection of glands cells called Mehlisí gland, which is believed to lubricate the uterus for egg passage. In addition, they possess a canal called Laurer's Canal, which leads from the oviduct to the dorsal surface of the body. Most trematodes possess an ovicapt, an enlarged portion of the oviduct where it joins the ovary. It probably controls the release of ova and spaces out their descent down the uterus

Digestive System

The great majority of digenetic trematodes are inhabitants of the vertebrate alimentary canal or its associated organs, especially the liver, bile duct, gall bladder, lungs, pancreatic duct, ureter and bladder. These are organs containing cavities rich in potential semi-solid food materials such as blood, bile, mucous and intestinal debris. Most species possess a mouth and forked, blind ended digestive system and feed actively. They are also capable of direct nutrient uptake through the tegument.

Nervous System

Paired ganglia at the anterior end of the body serve as the brain. From this nerves extend anteriorly and posteriorly. Sensory receptors are, for the most part, lacking among the adults, although they do have tangoreceptor cells. Larval stages have many kinds of sensory receptors, including light receptors and chemoreceptors. Chemoreception plays an important role in the free-living miracidial larvae recognising and locating its host.

Life Cycles

Eggs leave the host in faeces and are either eaten by a snail (or in very rare cases, by an Annelid worm) in which they hatch. In some species, eggs hatch in the water and become a ciliated free-swimming larva called the miracidium.

If it is a free-swimming miracidium it must penetrate the snail host through the body wall. Soon after penetration, the larva discards its ciliated epithelium and metamorphoses into a simple sac-like sporocyst. Germinal cells within the sporocyst develop into rediae. These mature and emerge from a birth pore or are released by rupture of the sporocyst

Each germ cell in the redia develops into a cercaria. Cercariae leave the snail host and are propelled through the environment by a whip-like tail. Cercariae usually penetrate the body wall of a second intermediate host and undergo further development. The fully developed, encysted metacercaria is infective to the definitive host. Infection of the definitive host occurs when the intermediate host is preyed upon and the metacercaria is released.

Development in the definitive host can occur once the cercariae have penetrated the hosts body, or, for those trematodes that have metacercariae, once the metacercariae excyst in the definitive hostís gut after being ingested. A variety of mechanisms can lead to excystation, including host enzymes, temperature, etc. Once excystation has occurred, the worms migrate to their appropriate location in the definitive host and the life cycle repeats.

Human Digenean Infections

Only about 12 of the 6,000 known species are infectious to mankind, but some of these species are important diseases with of 200 million people infected world wide. The species that infect humans can be divided into groups, the Schistosomiasomes and the non-Schistosomiasomes.


The Schistosomiasomes are all parasites of the circulatory system of their primary host, meaning they live and feed inside the blood vessels. Because of this they are all very thin animals, ranging in size from 10 to 30 mm (0.43 to 1.26 ins) in length to 0.2 to 1.0 mm in diametre. The males are shorter and thicker than the females. Females only reach sexual maturity after they have been mated by a male, the male has a long groove along one entire side of his body that he uses to clasp the female. After mating the two remain locked together for the rest of their lives. they can live for several years and produce many thousands of eggs.

Four species of Schistosomiasis are found to effect human beings, they are all members of the genus Schistosoma and all have snails as the intermediate host, S. mansoni is the most common while S. japonicum represents the greatest problems in control because it infects a large number of non-human mammals such as Cattle, Dogs and Rats.

Human Schistosomiasomes
Scientific Name Snail Genera Endemic Area
Schistosoma mansoni Biomphalaria spp. Africa, South America, Caribbean, Middle East
Schistosoma haematobium Bulinus spp. Africa, Middle East
Schistosoma japonicun Oncomelania spp. China, East Asia, Philippines
Schistosoma intercalatum Bulinus spp Africa


There seven major species of non-Schistosomiasomes which infect mankind, all infect people when they are eaten as metacercarial cysts - all leave in human faeces. One species, Paragonimus westermani lives in the lungs, and so can also have its eggs passed out in saliva. The other six species live in the human digestive system. Normally infection with these parasites is mildly to (occasionally) seriously unpleasant, only very rarely is it life-threatening.

Human non-Schistosomiasomes
Scientific Name Snail Genera Mode of Infection Endemic Area
Fasciolopsis buski Segmentina sp. Plants Asia, India
Heterophyes heterophyes Pirinella Mullet, Tilapia Asia, Eastern Europe, Egypt, Middle East
Metagonimus yokogawaii Semisulcospira sp. Carp, Trout Siberia
Gastrodiscoides hominis Helicorbis sp. Plants India, Vietnam, Philippines
Clonorchis sinensis Bulinus sp. Fish East Asia
Fasciola hepatica Lymnea sp. Plants Central America, South America
Paragonimus westermani Oncomelania sp. Crabs, Crayfish Asia

Important Publications

Key to the Trematoda, vol.1 Gibson, D.I., Jones, A., and Bray, R.A. (2002) ISBN 0851995470

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