Trematoda, a class of worms, including the "Flukes" (q.v.). They are characterised by the fact that the body consists of a single segment, of a flattened leaf-like-form, provided with suckers, and generally, also, hooks. The mouth leads into a branched alimentary canal, but there is no anus. The nervous system is on the dorsal side of the animal. The animals are hermaphrodite, with one exception, The classification of the order is follows :-
I. MONOGENETIC - development direct.
1. Tristomece. - Usually three suckers, e.g. Tristomum.
1. Polystomece. - Numerous suckers, e.g. Polystomum, Apidogaster.
II. DIGENETIC - animals pass through a metamorphsis.
1. Monostomidae. - One sucker.
2. Distomidae. - Leaf-like bodies, e.g. Distomum (Liver Fluke), Bilharzia (the cause of the disease known as haematuria).
3. Gasterostomidae. - Of which the larvae (Bucephalus) live in oysters, &c.
Holostomidae, e.g. Holostomum, which is parasitic in birds.