Infection by trematodes can cause disease in all five traditional vertebrate classes: mammals, birds, amphibians, reptiles, and fish.

This term can be traced back to the Old English name for flounder, and refers to the flattened, rhomboidal shape of the organisms.

For example: Trematodes are flattened oval or worm-like animals, usually no more than a few centimeters in length, although species as small as 1 millimetre (0.039 in) are known.

[12] The body surface of trematodes comprises a tough syncytial tegument, which helps protect against digestive enzymes in those species that inhabit the gut of larger animals.

[12] Although the excretion of nitrogenous waste occurs mostly through the tegument, trematodes do possess an excretory system, which is instead mainly concerned with osmoregulation.

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.

Trematodes generally lack any specialized sense organs, although some ectoparasitic species do possess one or two pairs of simple ocelli.

[12] Body wall musculature: Formed of three different muscle layers: circular, longitudinal, and diagonal.

In trematodes, the oral sucker is linked to the pharynx via a canal composed of meridional, equatorial, and radial muscle fibers.

Sperm cells travel from the seminal vesicles through the uterus to reach the ootype (the dilated distal part of the oviduct), where fertilization occurs.

After the egg is surrounded by yolk, its shell is formed from the secretions of Mehlis' glands, the ducts of which also open into the ootype.

From the ootype, the fertilized egg then travels back into the uterus, and is ultimately released from the genital atrium.

Eggs shed in water release free-swimming larval forms (Miracidia) 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 rediae and cercariae develop from the larvae which are then released and encyst as metacercariae, for instance on aquatic plants.

In humans or grazing animals, the metacercariae complete their life cycle and become full grown liver flukes.

One species of tremtaoda, Haplorchis pumilio, has evolved eusociality involving a colony of them creating a class of sterile soldiers.

They’re found in the highest numbers in the basal visceral mass, where competing trematodes tend to multiply during the early phase of infection.

This strategic positioning allows them to effectively defend against invaders, similar to how soldier distribution patterns are seen in other animals with defensive castes.

[16] Reflecting on their use for understanding the evolution of animal social castes, one review commented, "trematodes are a lineage for sociobiologists to keep a careful watch on!

"[17] Trematodes can cause disease in many types of vertebrates, including mammals, birds, reptiles, and fish.

These infections lead to a reduction in milk or meat production, which can be of significant economic importance to the livestock industry.

[21] Praziquantel is effective in the treatment of all diseases caused by flukes (clonorchiasis, dicrocoeliasis, echinostomiasis, fasciolopsiasis, fasciolosis, gastrodiscoidiasis, heterophyiasis, metagonimiasis, opisthorchiasis, paragonimiasis, and schistosomiasis).