Movement without muscles: Zoologists are on the trail of the evolution of body contractions

All animals move -- cheetahs faster, snails more slowly. Muscle contractions are the basis of all movements, at least according to general opinion. But there are animal groups, that don't have any muscles at all, as they branched off from the evolutionary path before muscle cells evolved. However these animal groups, for instance the sea sponges, are not immovable. Sponges are able to contract without muscles. These contractions were already known to sponge divers in ancient Greece, as Aristotele described in 350 BC. But which cells in the sponges are contracting?

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