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