Cothonozoa

From Tlaquanaru

The sea jellies of Tlaquanaru, form the phylum Cothonozoa, or "bell animals," are among the most ancient of Tlaquanaru's diploblastic metazoans. Like Terran cnidarians, the average cothonozoans are predatory members of either planktonic or benthic communities, and their bells can range from anywhere from a few millimeters to over a meter in diameter. Cothonozoans have an incomplete digestive tract, and are, for the most part, mostly stomach and reproductive organs, with or without tentacles covered in stinging cells (cnidaricysts).

It must be noted, and stressed that Tlaquanaran cothonozoans are not classified according to whether the polyp or medusa stage is dominant, but rather, the number of septae the gastric chamber is divided into. The three recognized classes of Cothonozoa all have species with polyp, or medusa stages dominant.

The stomach, or "gastric chamber," is a large sac-like chamber that may or may not be separate from the bell. Those cothonozoans which have the gastric chamber separate from the bell invariably follow a "cup and saucer," or a "goblet and bowl" body plan, which has evolved numerous times in almost all of the known groups. The gastric chamber is often divided up into multiple chambers through evagination, with further sub-dvisions and elongations of these evaginations, or "septae," form a labyrinth of food-storage chambers. The number of divisions forms the most basic means of taxonomy for cothonozoans, with three main groups, the Triseptata, which have septae in multiples of 3, the Tetraseptata, which have septae in multiples of 4, and the Pentaseptata, which have septae in multiples of 5.

It was originally thought that those living cothonozoans which have only one septa, such as Cylindera, and Cyclamphora, were the most primitive of all cothonozoans. However, recent thorough DNA/RNA hybridization and protein comparison assays strongly suggest that the single septa is actually a highly-derived condition, and that these single septa cothonozoans are actually descended from multiple-septae ancestors.

However, this is not to say that the cothonozoans never had a single septa to begin with. 750 to 700 million year old fossils suggest that the ancient cothonozoans, such as Protocylindera, Eocothonomedusa, and Archaeocothono, had a simple, relatively undifferentiated septum, and that differentiation of the septa occurred three separate times in their evolutionary history.

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