This group of tapeworms is of no medical importance but may cause pathology in heavily infected fish.
Adults are in the intestine of marine and freshwater fish. Eggs are eaten by a crustacean first intermediate host and develop to procercoids. When eaten by a fish second intermediate host, the procercoids begin progenetic development. After ingestion by a fish definitive host maturation of the parasite is complete.
These tapeworms of fish possess multiple internal sets of reproductive organs, but no external division into proglottids. The scolex is usually cup-shaped.