Trematods have fastening organs, several suckers and hooks.  Suckers are integumentary appendices in which the circular muscle structure has a more or less radial position.  The contraction of the sucker creates a depression that causes the adherence of the organ to its support. The mouth, terminal or subterminal, opens in the centre of a sucker.  The hooks only exist on direct development trematods and are implemented around the posterior suckers.

The parenchyma of trematods contains polyedric cells, rich in glycogen and fat. Regarding anaerobic species (parasites of the liver, small intestine and other bowels), glycogen is used during the fermentation that provides breething material; it divides into CO2 and fat acid, which is in the origin of fat.

The reproduction organs are hermaphrodite.

Trematods are divided in two groups that present different morphologies and cycles:

A. Monogenetic trematods B. Digenetic tramatods
Besides the mouth sucker, they have several posterior suckers generally provided with hooks.  Their development is direct and occurs in a single host. There are some exceptions but the majority use cold blooded Vertebrates as hosts (frogs for example). They only possess two suckers, one on the mouth and the other one ventral or terminal.  Sometimes these suckers disappear (case of blood trematods).  All have a complex cycle encompassing several generations and the passage through several hosts.  Larva phases always occur in a Mollusc. A waiting stage encysts either in a provisionnal host or in a foreign body.  The adult lives in a vertebrate bowels (mostly intestine and related organs).

Three examples of digenetic trematod cycle: Fasciola hepatica, Paragonimus ringeri and Schistosoma haematobium

One of the best known cycles is the one of Fasciola hepatica - the liver fluke; quite special as the waiting phase does not occur on a second host, but on aquatic plants.

The adult (about 3 cm) lives rolled up in the hepatic channels of Ruminants (mainly sheep).  Self-fecundation does not seem to be a rule and it is reciprocal during mating.  The eggs are in fact cocoons, as they contain a germ-cell (the egg itself) and several hundred viteline cells.  They fall in the intestine and are expelled with the animal faeces. If the eggs reach water, they continue their evolution.  The larva they turn into escapes from the cocoon by lifting the polar operculum and vigourously swims thanks to its tegument cilia: the larva is called miracidium.  It is attracted to about all species of  Limnaea (Pulmonate gastropod mollusc), mainly Limnaea trunculata.  It penetrates the mollusc lung and transforms into a sporocyst by degenerating all its organs except the teguments (which loose their cilia) and the reproductive cells.

In this bag, the germinal cells start to act and form larva that possess a embryo of digestive tube and masses of germ-cells. The sporocyst bursts and the larvae are liberated and migrate to the hodt hepato-pancreas. If the temperature is low, they generate other rédies, the daughter-larvae. If the temperature rises, they generate a new type of larva, the cercaria in which one can distinguish a young trematod with suckers, digestive tube and motive tail. The cercaria gets out of the limnaea, opening its way through the bowels.  After a short period of free life, it fastens to an aquatic plant  near the water surface, it looses its tail and secretes a strong cystic membrane; becoming then the infesting  metacercariaeThe ruminant, when it ingests this plant, will contaminate itself when plants come out of the water.  The cystic membrane is digested in the duodenum; the small metacercaria makes its way through the hepatic channels, and makes itself at home where it will reach sexual maturity.

This cycle often encompasses a third host, its the case for Man's lung fluke (Paragonimus ringeri) that can be found in the Far-East.  The Miracidium penetrates a Gastropod type Melanoïdes, the cercariae that result from this encyst in a Freshwater crabMan will contaminate itself when ingesting the crab if the latter is insufficiently cooked.  The métacercaire that is liberated drills through the intestinal wall, falls in the abdominal cavity, crosses the diaphragm and finally penetrates the lungs without passing through the blood vessels. The adults can be found in pairs in the bronchioles that dilate and become thicker to turn into a sort of cyst.

The metacercaire stage may be by-passed.  For example, the Schistosoma, also called Bilharzia, does not pass through this stage.  The species of this gender live in the breathing organs of Mammals  and all have separate sexes.  Several also infest Man.  This is the cycle of Schistosoma haematobium: the adults live in veins; for a long time the male hosts the female, very filiform, in a fissure in its ventral side.  Fecundated females abandon the males and go to the small veins of the bladder.  Thanks to a  subterminal spur, the eggs tear up the capillaries and fall in the vesical cavity; They are then driven to the exterior by the urines. The Miracidium penetrates a small Pulmonate: Bullinus, Physa or Planorbis.  The sporocyst develops by budding son-sporocysts that directly generate cercariae.  The latter leave their host and swim with their bifidous tail.  they directly penetrate Man's skin when the latter is in contact with water, they fall in the capillaries and are carried by the blood circulation.  They transform into adult worms inside the veins.