Sponges and placozoans seem simpler than comb jellies because they lack such nerve and muscle cells, and because they have simpler body plans. These nerve and muscle cells are diffusely arranged in a net, radially symmetrical around the comb jelly’s gravity-sensing organ. The comb jelly, it turns out, has some of the same complex nerve and muscle cell types as multicellular animals with bilateral symmetry. In the 1980s a species of comb jelly wreaked havoc in the Black Sea by consuming so many anchovy fish eggs that the fishing industry has yet to fully recover. Known as a sea walnut due to its shape and slow motion, the comb jelly is actually a fairly invasive species when inadvertently introduced into ecosystems lacking its ordinary competitors. Comb jellies also, like sponges and jellyfish and placozoans, lack brains. It looks like a jellyfish but lacks stinging cells. There are around 200 species of this marine creature. The comb jelly is named for its eight rows of linked cilia that look like combs and propel it through the water. Contenders include other jellyfish-like animals and placozoans-a sort of flat ciliated multicellular marine algae-eater aptly nicknamed “a sticky hairy plate.” Now, Joseph Ryan and colleagues, reporting in Science that they have sequenced the genome of the first comb jelly-a species called Mnemiopsis leidyi-say they wish to re-write the way early animal evolution is supposed to have happened. The sponge-with its deceptively simple design-has long been a leading candidate. 1 At the Foot of the Animal TreeĮvolutionists trying to trace the lineage of all animals back to its base have long debated what sort of organism is the most primitive. Image by Stefan Siebert/Brown University, via Science. However, evolutionary researchers believe the comb jelly’s genome shows that its kind is a more primitive ancestor of multicellular animals than sponges. Because comb jellies have nerve and muscles cells, they seem more complex than sponges. Eight rows of linked cilia, resembling combs, propel about 200 species of these organisms through marine ecosystems all over the world. It resembles a jellyfish but lacks stinging cells. This is a comb jelly, also known as a sea walnut.
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