A soft-bodied organism, the jellyfish Chrysaora melanaster

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The term is used to describe animals without skeleton, roughly corresponding to the group Vermes as proposed by Carl von Linné. All animals have muscles, but since muscles can only pull, never push, a number of animals have developed hard parts that the muscles can pull on, commonly called skeletons. Such skeletons may be internal, as in vertebrates, or external, like in arthropods. However, a surprising number of animals groups do very well without hard parts. This include animals like earthworms, jellyfish, tapeworms, squids and an enormous variety of animals from almost every part of the Animal Kingdom.

With few exceptions, soft-bodied animals are small. However, they do make up the majority of the animal biomass. If we were to stuff all animals on Earth with hard parts in one pile and the soft-bodied ones in another, estimates indicate that the pile of soft-bodied animals would be at least twice the size of the other pile, quite possibly much larger.

The lack of hard parts in Soft-bodied organisms makes them extremely rare in the fossil record. Accordingly, the evolutionary history of many of the soft-bodied groups are poorly known. The first major find of fossil soft-bodied animals was from the Burgess Shale in Canada. Today, several Burgess shale type faunas are known, but the history of the soft-bodied animals is still poorly understood.