Diphyllobothrium

Adults live in the intestine of a fish-eating bird or mammal. Unembryonated eggs are shed and pass out in the feces. Development takes a few days and a coracidium hatches from the egg. The coracidium, which can live for a few days, swims until being eaten by a copepod first intermediate host. The oncosphere penetrates the gut wall of the copepod and enters the hemocoele. After about 3 weeks it has developed into a procercoid, and is now infective.

When the copepod is eaten by a variety of fish second intermediate hosts, the procercoid penetrates through the gut wall and enters the musculature. There it begins a process of growth and differentiation into a plerocercoid, a process that takes many weeks. Some species do not develop in the muscles, but remain in the body cavity and are encapsulated there by a host response.

When infected fish are eaten by a definitive host, the plerocercoid is liberated and attaches to the intestine and matures. If infected fish are eaten by other fish, the plerocercoid may be able to migrate to the tissues of the new fish and remain as a plerocercoid. Thus, fish may be a paratenic host in this life cycle.