Department of Natural Resources and the Environment

College of Life Sciences and Agriculture

University of New Hampshire


Questions regarding relationships within polyclads and of polyclads with other flatworms abound. In fact, there is an on-going debate regarding the relationships of all flatworms in general with the remaining animals. Historically, the phylum Platyhelminthes has been considered one of the most primitive animal groups (left panel on figure). Some of the arguments for such a position included a solid body construction (referred to as acoelomate; “a” = without, “coelom” = body cavity) where parenchymal tissue fills the space between the gut and the body wall, a blind-ending digestive system, and no circulatory or respiratory systems.

two phylogenetic trees

With the advent of techniques that facilitate obtaining and analyzing large numbers of DNA sequences, many of the more traditional views of evolutionary relationships among animals have been changed. These changes also have affected the position of the flatworms. Two new subgroups have been erected, namely the Ecdysozoa which include all animals that molt, and the Lophotrochozoa. In these new evolutionary trees, flatworms are now included among the Lophotrochozoa, placing them much closer to annelids and mollusks (right panel of figure).

We know from morphological and molecular studies that polyclads are part of a lineage within the Platyhelminthes called the Rhabditophora. This lineage represents the largest group of flatworms and is named for the epidermal structures called rhabdites that characterize the worms (“rhabdito” = rhabdite granule, “phora” = bearing). The Rhabditophora contains many other free-living flatworms plus all the parasitic ones, i.e., tapeworms and flukes. Within the Rhabditophora, it is the order Macrostomida that contains the immediate relatives of the polyclads.

Relationships within the Polycladida are not well known. The group has been divided into two subgroups distinguished from each other by either having a ventral sucker (Cotylea) or not having a sucker (Acotylea). Although such a grouping has great intrinsic appeal, some families may be included in either subgroup. Additional problems are presented when animals are improperly collected or preserved. For example, the sucker may be broken off while an animal is removed from the substrate, or with improper fixation, the sucker may disintegrate. Classifications within the two suborders rely on morphological structures such as the arrangement of eyes and the reproductive systems. However, in many genera, the reproductive systems are relatively homogeneous with little differentiation. In such cases, other characters (eg, body shape, color, color pattern, pharynx structure, nuchal tentacles) are used for species identifications.

A relatively new and potentially very powerful approach to the classification of polyclads is the use of DNA sequence data. Preliminary studies have shown that polyclads can reliably be separated into species using DNA sequences. These studies only represent the beginning of an exciting new era in polyclad systematics. We now have in hand, the tools that allow us to establish independent tests of phylogenetic hypotheses that have been (or still will be) proposed for these amazing worms.

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These pages are for educational purposes only. If you use any of the information or the images in your classes, please credit these web pages. © MK Litvaitis, 2005

Photo credits to Leslie Newman, Andrew Flowers, or Anne DuPont, unnless otherwise indicated.


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