softbodied

Softbodied is a biological descriptor used for animals whose bodies lack rigid, mineralized skeletons or external shells. In contrast to hard-bodied organisms, soft-bodied animals rely on flexible tissues, muscles, and often hydrostatic pressure to maintain form, move, and interact with their environment. The term is commonly used in invertebrate biology to emphasize anatomy and evolution rather than taxonomy.

Anatomy and physiology without rigid skeletons allow diverse strategies for support and movement. Many soft-bodied animals use hydrostatic skeletons, with fluid-filled compartments that resist compression when muscles contract. Others rely on elastic bodies, muscular hydraulics, or protective mucus layers. This flexibility can enable burrowing, crawling, swimming, and squeezing into confined spaces, but often increases vulnerability to desiccation and predation.

Taxonomic scope and examples span several phyla. Cnidarians such as jellyfish and sea anemones have soft, flexible bodies. Flatworms and nematodes are also soft-bodied, as are many annelids (earthworms, leeches). Mollusks like slugs and many cephalopods (octopuses, squids) have highly muscular, non-skeletal bodies, though some mollusks retain shells. Arthropods, by contrast, possess hard exoskeletons and are not considered soft-bodied.

Fossil record and preservation are challenging for soft-bodied organisms since soft tissues decay rapidly. Exceptional fossil deposits, or Lagerstätten, such as the Burgess Shale and Chengjiang, preserve soft-bodied forms and have been crucial for understanding early animal evolution and the diversity of body plans that preceded mineralized skeletons.