If we were studying invertebrate zoology about 75 years ago this lecture would be about the Phylum Radiata and it would have included one other taxon, the Echinodermata. At that time radial symmetry was considered one of the most important physical characters used to group animals and the phylum designation for animals is testimony to that. All of this was based on the assumption that the first animals in the primordial oceans were surrounded by single celled photosynthetic unicellular organisms on which they fed. There would have been a distinct advantage for radial symmetry
Our oceans are filled with an amazing variety of little pelagic beasts and swimming among them are many with bilateral symmetry including one that was to be identified as the larval stage of the echinoderms. Well to make a long story short that was the end of the Radiata and the birth of the Phylum Coelenterata with the Cnidaria and Ctenophora as subphyla. To shorten the story even further the phylum designation Coelenterata has been dropped and each of the subphyla have been raised to Phyla.
Cnidarinas were one of the phyla that we examined closely in BIO2125; it was one of the few phlya in that course that we studied to the level of class. Certain things should come to mind when we revisit these old friends;
New to us this year is the question of how the the Cnidarians fit into the evolutionary scheme of the metazoans. In particular which is the ancestral form of the phylum, the medusa or the polyp? Some would suggest that the medusa, because it is radially symmetric and how it produces the gametes both suggest that these are primitive characteristics. Others would disagree citing the biradial symmtery of the anthozoa and the number and types of cnidocytes that they have.
Ctenophora are a new radially symmetric taxon for us to study and because it was historically included in the now defunct Radiata and Coelenterata phyla it has have always been assumed that the Ctenophora and Cnidaria are closely related to each other. These amazing little beasts swim through the oceans with a pair of tentacle dragging behind them capturing prey using their unique colloblast cells. The octaradially arranged ctene, formed by the fusion of cilia, propel them through the oceans where their unique bioluminescence allows potential mates to find each other.
But there are a number of ctenophoran characteristics that raise some questions about just how closely they are related to the Cnidaria. We've already mentioned cilia compared to the monciliate condition of the Cnidaria. In addition to this is a complete alimentary tract, apparently mesodermal muscle in the retractable tentacles and a very distinct biradial symmetry. Some believe that ctenophorans represent a radial experiment from a ciliate metazoan ancestor making the Cnidarians another evolutionary dead-end. Others believe that the culmination of the shift from radial to biradial culminates is the Ctenophora.
(updates are in red,
BIO3334 Alternation of generations, Apical organs. Athecate, Blastostyle, Cnidoblast, Coenosarc, Colloblast cells, Comb plates, Comb rows, Complete septa, Contractile hypothesis, Ctene, Dactylozooids, Dimorphic life cycle, Exumbrella, Gastric pouches, Gonophores, Hydranth, Hydrocalus, Hydrorizia, Incomplete septa, Interstitial cells, Manubrium, Myoneme, Osmotic hypothesis, Pedal disk, Pedal laceration, Perisarc, Pneumatophore, Ptychocysts, Radial canals, Retractor muscles, Rhopalium, Ring canals, Spirocyst, Statolith, Subumbrella, Tension hypothesis, Thecate, Trilobed mesenterial filaments, Velum.
*Not all of the lecture slides may be presented in class! This handout is in the Adobe Acrobat PDF format. Use this link to get a copy of the Acrobat Reader from Adobe (It's free!)
(Log into your Digital Diversity eBook before using these links)
Radiata/Coelenterata (Cnidaria and Ctenophora
Websites you might find interesting
© Jon G. Houseman. Permission required to reproduce or display this material