These are not of medical importance, but many species are associated with pronounced lesions at the site of their attachment to the intestine of their fish hosts.
Most species are intestinal parasites of freshwater fish and use aquatic oligochaetes as intermediate hosts. One species is believed to be progenetic, eliminating the fish and maturing in the coelom of the oligochaete while still retaining some of its larval characteristics. It may be that the unusual lack of segmentation in these tapeworms is an indication that all its species exhibit some degree of progenesis. Very few life cycles of caryophyllidean cestodes have been completed experimentally. It seems clear, from abundant circumstantial observations, that those which infect fish typically develop into a procercoid in oligochaetes and simply complete their maturation in the gut of a fish, without requiring a second intermediate host.
The scolex is weakly developed. These distinctive, small (< 10 cm) tapeworms have only a single set of reproductive organs, and consequently no division into proglottids.