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Fascinating, elegant, and mysterious to watch in the water, take a jellyfish out of the water, and it becomes a much less fascinating blob. This is because jellyfish are about 95 percent water.

“Jellyfish” is a decidedly unscientific term—the creatures are not fish and are more rubbery than jamlike—but scientists use it all the same (though one I spoke with prefers his own coinage, “gelata”). The word “jellyfish” lumps together two groups of creatures that look similar but are unrelated. Scientists call them "jellies".

Lacking brains, blood, or even hearts, jellyfish are pretty simple critters. They are composed of three layers: an outer layer, called the epidermis; a middle layer made of a thick, elastic, jelly-like substance called mesoglea; and an inner layer, called the gastrodermis. An elementary nervous system, or nerve net, allows jellyfish to smell, detect light, and respond to other stimuli. The simple digestive cavity of a jellyfish acts as both its stomach and intestine, with one opening for both the mouth and the anus.

[Via NOAA]