|Botulus microporus, a giant digenean parasite from the intestine of a lancetfish|
Trematoda is a class within the phylum Platyhelminthes that contains three groups of parasitic flatworms, commonly referred to as "flukes". These groups of parasitic worms are Cestoda, Monogenea and Trematoda.
Taxonomy and biodiversity 
The trematodes or flukes are estimated to include 18,000 to 24,000 species, and are divided into two subclasses. Nearly all trematodes are parasites of mollusks and vertebrates. The smaller Aspidogastrea, comprising about 100 species, are obligate parasites of mollusks and may also infect turtles and fish, including cartilaginous fish. The Digenea, which constitute the majority of trematode diversity, are obligate parasites of both mollusks and vertebrates, but rarely occur in cartilaginous fish.
Formerly the Monogenea were included in Trematoda on the basis that these worms are also vermiform parasites, but modern phylogenetic studies have raised this group to the status of a sister class within the Platyhelminthes, along with the Cestoda.
Trematodes are flattened oval or worm-like animals, usually no more than a few centimetres in length, although species as small as 1 millimetre (0.039 in) (Monogenea) and as large as 7 centimetres (2.8 in)[which?] are known. Their most distinctive external feature is the presence of two suckers, one close to the mouth, and the other on the underside of the animal.
The body surface of trematodes comprises a tough syncitial tegument, which helps protect against digestive enzymes in those species that inhabit the gut of larger animals. It is also the surface of gas exchange; there are no respiratory organs.
The mouth is located at the forward end of the animal, and opens into a muscular, pumping pharynx. The pharynx connects, via a short oesophagus, to one or two blind-ending caeca, which occupy most of the length of the body. In some species, the caeca are themselves branched. As in other flatworms, there is no anus, and waste material must be egested through the mouth.
Although the excretion of nitrogenous waste occurs mostly through the tegument, trematodes do possess an excretory system, which is instead mainly concerned with osmoregulation. This consists of two or more protonephridia, with those on each side of the body opening into a collecting duct. The two collecting ducts typically meet up at a single bladder, opening to the exterior through one or two pores near the posterior end of the animal.
The brain consists of a pair of ganglia in the head region, from which two or three pairs of nerve cords run down the length of the body. The nerve cords running along the ventral surface are always the largest, while the dorsal cords are present only in the Aspidogastrea. Trematodes generally lack any specialised sense organs, although some ectoparasitic species do possess one or two pairs of simple ocelli.
Reproductive system 
Most trematodes are simultaneous hermaphrodites, having both male and female organs. There are usually two testes, with sperm ducts that join together on the underside of the front half of the animal. This final part of the male system varies considerably in structure between species, but may include sperm storage sacs and accessory glands, in addition to the copulatory organ, which is either eversible, and termed a cirrus, or non-eversible, and termed a penis.
There is usually only a single ovary, which is connected, via a pair of ducts to a number of vitelline glands on either side of the body, that produce yolk cells.[clarification needed] Eggs pass from the ovary into a glandular receptacle called the ootype or Mehlis' gland, where fertilization occurs. This opens into an elongated uterus that opens to the exterior close to the male opening. The ovary is often also associated with a storage sac for sperm, and a copulatory duct termed Laurer's canal.
Life cycles 
Almost all trematodes infect mollusks as the first host in the life cycle, and most have a complex life cycle involving other hosts. Most trematodes are monoecious and alternately reproduce sexually and asexually. The two main exceptions to this are the Aspidogastrea, which have no asexual reproduction, and the schistosomes, which are dioecious.
In the definitive host, in which sexual reproduction occurs, eggs are commonly shed along with host feces. Eggs shed in water release free-swimming larval forms that are infective to the intermediate host, in which asexual reproduction occurs.
A species that exemplifies the remarkable life history of the trematodes is the bird fluke, Leucochloridium paradoxum. The definitive hosts, in which the parasite multiplies, are various woodland birds, while the hosts in which the parasite grows (intermediate host) are various species of snail. The adult parasite in the bird's gut produces eggs and these eventually end up on the ground in the bird's faeces. Some very fortunate eggs get swallowed by a snail and here they hatch into tiny, transparent larva (miracidium). These larvae grow and take on a sac-like appearance. This stage is known as the sporocyst and it forms a central body in the snail's digestive gland that extends into a brood sac in the snail's head, muscular foot and eye-stalks. It is in the central body of the sporocyst where the parasite replicates itself, producing lots of tiny embryos (redia). These embryos move to the brood sac and mature into cercaria.
Human infections are most common in Asia, Africa, South America, or the Middle East. However, trematodes can be found anywhere where human waste is used as fertilizer. Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) is an example of a parasitic disease caused by one of the species of trematodes (platyhelminth infection, or "flukes"), a parasitic worm of the genus Schistosoma.
- Tissue flukes infect the bile ducts, lungs, or other biological tissues. This group includes the lung fluke, Paragonimus westermani, and the liver flukes, Clonorchis sinensis and Fasciola hepatica.
- Blood flukes inhabit the blood in some stages of their life cycle. Blood flukes include species of the genus Schistosoma.
- Littlewood, D T J; R. A. Bray (7 December 2000). "The Digenea". Interrelationships of the Platyhelminthes. Systematics Association Special Volume 60 (1 ed.). CRC. pp. 168–185. ISBN 978-0-7484-0903-7.
- Poulin, Robert; Serge Morand (7 January 2005). Parasite Biodiversity. Smithsonian. p. 216. ISBN 978-1-58834-170-9.
- Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 230–235. ISBN 0-03-056747-5.
- Examples of the use of this term:
- Aspidogastrea knol I
- Aspidogastrea knol II
- Aspidogastrea knol III
- Trematoda knol