Medusa & polyp stages of cnidarians. Credit: Luc Viatour & Peter J. Bryant
True Jellies & Corals: Phylum Cnidaria
And, finally, we have gotten to the group we are interested in. Within this large and diverse phylum we have many familiar forms: soft corals, stony corals, anemones, box jellies, the “true” jellies, Hydra, hydroids, sea pens, sea whips, sea pansies. Making sense of all this diversity can be a challenge, and before we delve into it, we need to better understand what defines a cnidarian. The name is derived from the Greek cnidos, meaning “stinging nettle”—an apt name for a group associated with its often painful stings. The manner with which it stings is unique in the animal kingdom. Within certain modified neural cells are specialized organelles (nematocysts) under osmotic pressure, capable of deploying a venomous barb when contacted. This feature alone is enough to identify any cnidarian, but also worth mentioning is their radial symmetry and gelatinous matrix (in place of a legitimate middle tissue layer). These last two features are shared with the comb jellies, but this is perhaps best understood as the retention of an ancestral jelly-like condition. To reiterate, these two groups of “jellies” are only distantly related. The radial symmetry of jellies makes them seem particularly foreign to us bilaterally biased creatures, but research indicates that the same signaling pathways that give us our anterior/posterior axis are potentially responsible for the oral/aboral axis of cnidarians. Furthermore, the tetraradial symmetry of this phylum is only seen in the medusozoans to be discussed next; anthozoans (wherein resides all the various coral groups) are actually bilaterally symmetrical like us. So it seems quite plausible that cnidarians could have originally possessed a poorly-developed form of bilateral symmetry. While jellies may seem far removed from a species as morphologically complex as ourselves, it’s important to realize that much of the genetic architecture that enables that complexity can be found in a nascent stage here with the jellies. Much of our basic physiology—from how our nerves communicate to how our cells adhere together—can be seen as cnidarian in nature. True Jellies: Subphylum Medusazoa This group includes all of the pulsating jellies (scyphozoans), the enormously diverse hydrozoans, as well as the box jellies (cubozoans), the unusual stalked jellies (staurozoans), and even some bizarre internal parasites (polypodiozoans and myxozoans)! With the exception of the parasitic forms, all these groups possess a dual stage life cycle, alternating between an asexual polyp and a sexual medusa (i.e.… More:
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