These tiny creatures are called hydromedusae, and are shown here about 17 times larger than life-size. Hydromedusae are among the simplest multicellular organisms. These examples live in the surface waters of the Atlantic, off northwest Africa (they were collected using a net towed along at the water's surface). They propel themselves through the water with rhythmic contractions of the swimming muscles n their umbrella-shaped body, controlled by a ring of nerve fibers around the body rim.