|Platyhelminthes - "the flatworms"|
|This phylum's 'place'
relative to other phyla:
|Selected taxa within this taxon:
Platyhelminthes are a diverse group of
flatten worms with a fairly simple body plan
Are lack of systems and lack of coelom indications of primitive conditions or the result of constraints associated with selection for flatten morphology?
Traditionally considered the most primitive of the bilateria with the bilateria being grouped into acoelomates, psuedocoelomates, and coelomates.
Why were Platyhelminthes considered the primitive outgroup?
Now many zoologist are leaning toward three major branches of the bilateria where platyhelminthes and rotifers are closely related to lophotrochozoa based primarily on molecular similarities.
The acoelomate condition may be secondary:The traditional phyla groupings (e.g. the pseudocoelomates) may share these characteristics due to convergent evolution.
filled in as an adaptive response to reduce body size neotonous from stage prior to coelom development. All three conditions may have occurred in the development sequence of the original bilaterian ancestor.
LESSON: Do not assume simplicity is necessarily primitive.Could traditional thoughts on phylogeny been influence in part by "world views" of the way we thought nature ought to be?
For example, the Great Chain of Being, while discounted, has resulted in the idea of 'progress' being pervasive in modern civilization. This may have biased the traditional tree toward a more linear one (as compared with a more modern version).
However, there appears to be one group that may be the outgroup of the bilateria phyla (based on molecular evidence and supported by some morphological evidence): the Turbellarian order Acoels of the Platyhelminthes. If the Acoels are "the earliest branch within the bilaterian clade that left an extant should ancestor", should this group be considered as a separate phylum?
Class Trematoda - flukes
Fecundity is higher than that of free-living flatworms. Why?
Typically at least two hosts (the intermediate host is commonly a gastropod mollusc). In intermediate hosts numerous eggs produced by asexual reproduction. The definitive host (parasite is the sexual, adult form) is almost always a vertebrate.
Clonorchis - the human liver fluke.
- the human lung fluke.
Class Cestoda - tapeworms
phylogenetic trees of tapeworms (and
other parasites as well) often reflect the
phylogenetic trees of their hosts. Why? But
sometimes there a major descrepencies among these
Does this support evolutionary theory?
Pigs, beef and fish are often intermediate hosts for numerous tapeworm species and transmitted to humans by direct consumption. e.g. the beef tapeworm Taenia saginatus, can grow to a length of 10 metres, but does not generally pose a threat to health.
etcoparasites usually on skin and gills of fish. High
specificity both in terms of host species and site of
classes as well as the phylum may be paraphyletic.
Fossil records of Platyhelminthes are minimal.
Why might they not leave many trace fossils?
from the platyhelminthes:
Why might a group of organisms be "simple"? Can organisms evolve simplicity in some ways but not others?