|Ctenophora - comb jellies|
|This phylum's 'place'
relative to other phyla:
|Classes within this taxon:
are gelataneous marine animals, typically planktonic.
100-150 species but diversity is likely significant higher (Why?)
Why might radial symmetry work
well for a planktonic organism?
What do these traits suggest about a close evolutionary relationship to the cnidarians?
Class of ctenophores
parasitic form living in the body of the pelagic
the species Beroe,
roughly thimble shaped and opens up like a sack to engulf
The phylogenetic relationship of ctenophores to other phyla
Not well preserved as fossils (why?), Recently, two species have been found from the Late Devonian, and are quite similar to living ctenophores. Other ctenophore-like forms have been found in the Cambrian-age Burgess Shale and Chengjiang Formation.
Despite similarities to cnidarians, ctenophores appear more closely related to the bilaterian animals:
However, some molecular data has contradicted this view. Perhaps (also suggested by gross morphological similarities):
More recent molecular
data suggest that ctenophores may be the first
to split off from the lineage that gave rise to all other
animals, including sponges! Development
of the nervous
system seems to be radically different, supporting this
Ideas to consider from the ctenophores: