ABSTRACT
The cestodes are among the most diverse of the major platyhelminth groups. The adult stages of these obligate parasites tend to exhibit a fairly high degree of host specificity. This specificity is reflected to some extent in the current ordinal level organization of the class (see Khalil et al. 1994); the adults of most tapeworm orders parasitize members of one or two classes of vertebrates. That is not to say that ordinal membership is based entirely on adult host associations, because many orders are also defined by conspicuous, unique morphological features. Recent phylogenetic work, however, has begun to call into question the monophyly of at least some of the 14 currently recognized orders of cestodes (see Brooks et al. 1991; Hoberg et al. 1997c, 1999b; Mariaux 1998; Caira et al. 1999; Olson and Caira 1999). Particularly pivotal in these discussions are two of the tapeworm orders that parasitize elasmobranchs (the Tetraphyllidea and Lecanicephalidea) and one that commonly parasitizes freshwater bony fishes (the Proteocephalidea) because preliminary evidence suggests that at least some species in these orders have closest relatives that are currently members of one or more other cestode orders. If this is the case, the classification of cestodes cannot be revised to reflect monophyly until the relationships among and within these groups are more completely understood and a stable phylogenetic hypothesis has been established. One of the primary objectives of the present study was to further explore the relationships of these groups, concentrating on species in the two orders that parasitize elasmobranchs.
