Cysticercoid of Hymenolepis nana. In humans, Hymenolepis nana has a direct life cycle, with the cysticercoids developing in the villi of the small intestine, and auto infection is possible. In rodents, the life cycle may be either direct or indirect, with flour beetles or fleas serving as intermediate hosts.
Taenia hydatigena cysticercus in a sheep liver. When an appropriate intermediate host (any wild or domestic ruminant) ingests the egg of Taenia hydatigena, the egg will hatch in the small intestine and the released oncosphere will reach the liver via the blood. The embryo then breaks out of the portal vessels and migrates through the liver, eventually leaving the liver as a cysticercus and attaching to the serosal surface of organs and mesentery to await ingestion by a canid.
(also referred to as Pseudophyllidean) tapeworms
Adult Diphyllobothrium spp. occur in the small intestine of fish-eating mammals. Adults are 2-12 m long, yellowish-grey in color with dark central markings caused by the centrally located uterus and eggs. With an indirect life-cycle, Diphyllobothrium latum uses first a copepod to harbor the development of the coracidium to a procercoid, and then fresh-water fish as a second intermediate host where the pleurocercoid develops. These pleurocercoids accumulate in the fish and are then infective to mammals ingesting them.
In place of having suckers on the scolex, as many members of other classes of cestode have, Diphyllobothrium has two narrow, deep muscular groves called bothria. These bothria are often indistinct. It's name, a borrowing from the Greek di- (two) + phyllodes (like leaves) + bothrion (trench); reflects the morphology of its scolex.