Bucephalus is the genus name for many trematode flatworm

Flatworm

The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...

s that are parasites of molluscs and fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

. Like other Bucephalidae

Bucephalidae

Members of the family Bucephalidae are trematode flatworms with no oral sucker, using instead a rhynchus as an adhesive organ at their anterior end. Many are common parasites of freshwater fish....

, they are found in fish both as adults and as metacercariae

Trematode lifecycle stages

Trematodes are small parasitic flatworms that use vertebrates as their definitive host, and molluscs as their intermediate host. In order to accomplish this, they have several varied lifecyle stages....

. In marine and freshwater teleosts, they live as parasites inside the digestive tract, especially the intestine.

The genus Bucephalus was based on a the earliest known bucephalid, B. polymorphus

Bucephalus polymorphus

Bucephalus polymorphus is a species of the Bucephalidae family of Digenea, a subclass of Trematodes within the phylum Platyhelminthes. It is characterized by having a mouth near middle of body along with a sac-like gut. The adults occur in the centre of the ventral surface. The adults occur in the...

von Baer (1827), initially described from a cercaria larva. Von Siebold (1848) believed that the adult bucephalid he named Gasterostomum fimbriatum represented an adult form of the same bucephalid, but this identity has never been proven.

The name Bucephalus meaning "ox head" was chosen because of the horn-like appearance of the forked tail (furcae) of its cercaria. By what Manter calls a "curious circumstance", horns are also suggested by the long tentacles of adult worms.

They are distinguished from other genera in the same family by having tentacles associated with the anterior sucker. Genus members have their mouth in the middle of the body.

An earlier name for this genus was Gasterostomum, given by von Siebold in 1848 to all adult trematodes with a ventral mouth. Odhner (1905) established two suborders of digenean trematodes called Gasterostomata and Protostomata. The two genera in Gasterostomata were Gasterostomum (now Bucephalus) and Prosorhynchus, of which the former has an anterior sucker separate from its digestive tract and the latter has an anterior rhynchus. Members of the genus Bucephalus are also sometimes referred to as "gasterostomes."