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Ctenophores are gelataneous marine animals, typically planktonic.
100-150 species but diversity is likely significant higher (Why?)

• 'radially symmetrical’ and 'diploblastic' or bi-radial and triploblastic?

 

Why should plankton be radial?
 
 
 

 
• Eight "comb rows" of cilia for locomotion.

 
In oceanic forms, musculature more important than cilia in swimming. Why?

 
 
• No stinging cells, but instead sticky cells called colloblasts.

The spiral filament is contractile. Is this structure homologous to the nematocysts of cnidarians?

One group possesses nematocysts and had previously been an argument for close relationship with cnidarians, but now believed are "kleptocnidae"









• Mouth and anus w/ 4 digestive canals.
• No discrete respiratory organs or circulatory system.  Why?

• The nervous system is primitive, and though perhaps a bit more complex than Cnidaria.

• Statolith though different than that of cnidarians

Natural History

• Marine

• Typically planktonic, transparent without pigmentation (but not always).  Why transparent?
  

But one group is benthic
 
• Voracious predators preying on small zooplankton.  An introduction of one species (Mnemiopsis  leidyi) to Black Sea has resulted in major changes in its fisheries.

 
• Many ctenophores (like various other marine planktonic organisms) are bioluminescent

Usually blue-green

.
 
 
 
 
 





Not to be confused with iridescence, a light-scattering produced by beating of comb rows.

• simple life cycle (i.e. most are planktonic during entire life) and never colonial.

• Most are hermaphroditic.

• Egg and sperm released and fertilization is external, producing a cydippid larva, which is essentially a miniature of its parents.



What do these traits suggest about a close evolutionary relationship to the cnidarians?

Class of ctenophores

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.

Ctenorhabdotus of the Burgess Shale

Despite similarities to cnidarians, ctenophores appear more closely related to the bilaterian animals:

• pair of anal pores (complete digestive system).

• possess a third 'tissue layer' between the endoderm and ectoderm.

• Radial symmetry may be secondary.

However (beside gross morphological similarities), some molecular data has contradicted this view.  Perhaps:

More recent molecular data suggest that ctenophores may be the first phylum to split off from the lineage that gave rise to all other animals, including sponges!

Ideas to consider from the ctenophores:

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