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(formerly Coelenterata)
This phylum's 'place'
relative to other phyla:
  • Metazoan
    • Diploblast
Selected taxa within this taxon:
  • Class Hydrozoa
  • Class Scyphozoa
  • Class Cubozoa 
  • Class Anthozoa

Cnidarians are a diverse group of typically marine organisms with a fairly simple body plan: ~10,000 living species described.

Relatively early emergence in fossil record

Possible link to Ediacarian fauna Paleozoic corals though different coral taxa than modern reefs are composed. Soft-bodied cnidarians also know from Cambrian

A  flotilla of giant jellyfish marooned on a beach
500 million years ago unearthed in  Wisconsin


Similar to other phyla that are without bilateral symmetry and without three tissue layers.  Does sharing of primitive characteristics indicate that these phyla are more closely related to each other than to triploblastic, bilateral phyla?  If diploblasts are not a monophyletic group, is this evidence that ancestral triploblasts were diploblasts?

Body Plan
  (a body plan song)


Natural History

"A" Portuguese man-of-war is composed of
 various types of polyps for feeding,
 reproduction and bouyancy.

 Cnidarians have acquired a bi-morphic life-style associated with these stages:

- sessile polyp, which is attached to the substrate

- a free-swimming/floating medusa. How does this differ structurally from than the polyp stage?

Do all cnidarians alternate (metamorphosize) from one form to the other (both medusal and polyp stages)?  See below.

Major classes

Class Hydrozoa


Example of the class:



Class Scyphozoa
The true jellyfish

Example of the class: Aurelia aurita

Class Cubozoa
Small class in size and diversity and recently considered in a different order than Scyphozoa.
Class Anthozoa
 ‘flower animals’ contain majority of cnidarian species (speciose).



There is presently debate on whether Hydrozoa or  Anthozoa is the most primitive cnidarians:

 Ideas to consider from the cnidarians:
A particular body plan may constrain the evolutionary pathways that a phylum can take (i.e. the nervous system and symmetry; tissue layers and habitat).
Lecture Sources:
  • Pechenik. Jan A.  2000. Biology of the Invertebrates.  McGraw-Hill, New York.

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