These unusual parasites are primarily parasites of cartilaginous fishes and primitive bony fishes and are considered to be most closely related to the tapeworms, although they have some features which are more fluke-like. They are a relatively obscure group, of interest mostly for understanding the evolution of the Cestoda.
The life cycles of these parasites are incompletely known except for a very few species. The evidence is that they do have an intermediate host which may be a fish or invertebrate. The larval stage is the lycophore, which has some resemblance to the oncosphere of other cestodes.
The adult body is unsegmented. There is no gut. A scolex is absent but there may be an anterior sucker for attachment. There is a single set of male and female reproductive organs.
The lycophore larva is ciliated and has 10 hooks.