There are typically two suckers, an anterior oral sucker surrounding the mouth, and a ventral sucker sometimes termed the acetabulum, on the ventral surface. The oral sucker surrounds the mouth, while the ventral sucker is a blind muscular organ with no connection to any internal structure.
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. These terms are common in older literature, when they were thought to reflect systematic relationships within the groups. They have fallen out of use in modern digenean taxonomy.
While the sexual formation of the digenean eggs and asexual reproduction in the first larval stage (miracidium) is widely reported, the developmental biology of the asexual stages remains a problem. 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.
In addition, some digeneans possess a canal called Laurer's Canal, which leads from the oviduct to the dorsal surface of the body. The function of this canal is debated, but it may be used for insemination in some species or for disposal of waste products from reproduction in other species. 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.
The uterus typically opens into a common genital atrium that also received the distal male copulatory organ (cirrus) before immediately opening onto the outer surface of the worm. The distal part of the uterus may be expanded into a metraterm, set off from the proximal uterus by a muscular sphincter, or it may be lined with spines, as in the Monorchiidae and some other families.
The alternation of sexual and asexual generations is an important feature of digeneans. This phenomenon involves the presence of several discrete generations in one life-cycle.
A typical digenean trematode life cycle is as follows. Eggs leave the vertebrate host in faeces and use various strategies to infect the first intermediate host, in which sexual reproduction does not occur. Digeneans may infect the first intermediate host (usually a snail) by either passive or active means. The eggs of some digeneans, for example, are (passively) eaten by snails (or, rarely, by an annelid worm), in which they proceed to hatch. Alternatively, eggs may hatch in water to release an actively swimming, ciliated larva, the miracidium, which must locate and penetrate the body wall of the snail host.
After post-ingestion hatching or penetration of the snail, the miracidium metamorphoses into a simple, sac-like mother sporocyst. The mother sporocyst undergoes a round of internal asexual reproduction, giving rise to either rediae (sing. redia) or daughter sporocysts. The second generation is thus the daughter parthenita sequence. These in turn undergo further asexual reproduction, ultimately yielding large numbers of the second free-living stage, the cercaria (pl. cercariae).
Free-swimming cercariae leave the snail host and move through the aquatic or marine environment, often using a whip-like tail, though a tremendous diversity of tail morphology is seen. Cercariae are infective to the second host in the life cycle, and infection may occur passively (e.g., a fish consumes a cercaria) or actively (the cercaria penetrates the fish).
The life cycles of some digeneans include only two hosts, the second being a vertebrate. In these groups, sexual maturity occurs after the cercaria penetrates the second host, which is in this case also the definitive host. Two host life-cycles can be primary (there never was a third host) as in the Bivesiculidae, or secondary (there was at one time in evolutionary history a third host but it has been lost).
In three-host life cycles, cercariae develop in the second intermediate host into a resting stage, the metacercaria, which is usually encysted in a cyst of host and parasite origin, or encapsulated in a layer of tissue derived from the host only. This stage is infective to the definitive host. Transmission occurs when the definitive host preys upon an infected second intermediate host. Metacercariae excyst in the definitive host’s gut in response to a variety of physical and chemical signals, such as gut pH levels, digestive enzymes, temperature, etc. Once excysted, adult digeneans migrate to more or less specific sites in the definitive host and the life cycle repeats.
It is likely that more complex life cycles evolved through a process of terminal addition, whereby digeneans survived predation of their mollusc host, probably by a fish. Other hosts were added by until the modern bewildering diversity of life cycle patterns developed.
|Scientific Name||First Intermediate Host||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-schistosomes that infect humans are listed below. People become infected after ingesting metacercarial cysts on plants or in undercooked animal flesh. Most species inhabit the human gastrontestinal tract, where they shed eggs along with host feces. Paragonimus westermani, which colonizes the lungs, can also pass its eggs in saliva. These flukes generally cause mild pathology in humans, but more serious effects may also occur.
|Scientific Name||First Intermediate Host||Mode of Human 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, North America|
|Fasciola hepatica||Lymnea sp.||Plants||Central America, North America, South America|
|Paragonimus westermani||Oncomelania sp.||Crabs, crayfish||Asia|
Key to the Trematoda, vol.1 Gibson, D.I., Jones, A., and Bray, R.A. (2002) ISBN 0-85199-547-0 Interrelationships of the Platyhelminthes. D.T.J. Littlewood and R.A. Bray (2001) ISBN 0-7484-0903-3 Yamaguti, S. (1971) Synopsis of digenetic trematodes of vertebrates. Keigaku Publishing Co., Tokyo.