Pseudophyllidea van Beneden in Carus, 1863, a well recognised order of tapeworms (Platyhelminthes:
Eucestoda), is suppressed because it is composed of two phylogenetically unrelated
groups, for which the new names Bothriocephalidea and Diphyllobothriidea are proposed.
The new orders differ from each other in the following characters: (i) position of
the genital pore: on the dorsal, dorso-lateral or lateral aspects and posterior to
the ventral uterine pore in the Bothriocephalidea versus on the ventral aspect of
segments and anterior to the uterine pore in the Diphyllobothriidea; (ii) the presence
of a muscular external seminal vesicle in the Diphyllobothriidea, which is absent
in the Bothriocephalidea; (iii) the presence of a uterine sac in the Bothriocephalidea,
which is absent in the Diphyllobothriidea; and (iv) the spectrum of definitive hosts:
mainly teleost fishes, never homoiothermic vertebrates in the Bothriocephalidea, versus
tetrapods, most frequently mammals, in the Diphyllobothriidea, with species of Diphyllobothrium,
Spirometra and Diplogonoporus parasitic in humans. The Diphyllobothriidea, which includes
17 genera in four families (Digramma is synonymised with Ligula), is associated with
cestode groups that have a range of plesiomorphic characters (Haplobothriidea and
Caryophyllidea), whereas the Bothriocephalidea, consisting of 41 genera grouped in
four families, is the sister-group to the 'acetabulate' or 'tetrafossate' cestodes,
which are generally regarded as having derived characters.