PHYLOGENY OF THE ORDERS OF THE EUCESTODA (CERCOMEROMORPHAE) BASED ON COMPARATIVE MORPHOLOGY - HISTORICAL PERSPECTIVES AND A NEW WORKING HYPOTHESIS

The phylogeny of the Eucestoda was evaluated based on a suite of 49 bi nary and multistate characters derived from comparative morphological and ontogenetic studies; attributes of adult and larval tapeworms were considered a single most parsimonious tree (MPT) (consistency index = 0.872; retention index = 0.838; and homoplasy index = 0.527) was full y resolved and is specified by the following: (Gyrocotylidea, (Amphili nidea, ((Spathebothriidea, (Pseudophyllidea. ((Diphyllidea, (Trypanorh yncha, (Tetraphyllidea, (Lecanicephalidea. ((Nippotaeniidea, (Tetrabot hriidea, Cyclophyllidea)), Proteocephalidea))))), Haplobothriidea)ii, Caryophyllidea))). Monophyly for the Eucestoda was firmly corroborated . Trees derived from the primary and bootstrap analyses were congruent , but low values, particularly for relationships among the tetrafossat e tapeworms, indicated additional examination is warranted. The MPT wa s found to be the mast efficient hypothesis for describing character e volution and in specifying relationships among the orders when compare d to those concepts that had been developed for the tapeworms over the past century. Areas of congruence were shared among the current hypot hesis and one or more of the prior hypotheses. Major conclusions inclu de: (1) Caryophyllidea are basal and monozooy is ancestral; (2) difoss ate forms are primitive. and the Pseudophyllidea arts the sister group of the strongly polyzoic tapeworms; (3) Nippotaeniidea are highly der ived. (4) the higher tapeworms (Tetraphyllidea, Lecanicephalidea. Prot eocephalidea, Nippotaeniidea, Tetrabothriidea, and Cyclophyllidea) are closely related or potentially coordinate group (5) Terrabothriidea a nd the Cyclophyllidea are sister groups; and (6) Tetraphyllidea is par aphyletic, with the Onchobothriidae basal to the Phyllobothriidae. Cha racter support for placement of the Tetrabothriidea continues to be co ntradictory, and this order may represent a kev to understanding the p hylogeny of the higher cestodes. The current study constitutes a compl ete historical review and poses a new and robust hypothesis for the ph ylogeny of the Eucestoda.