Pehmeälihisiä

Pehmeälihisiä is a descriptive term in Finnish biology used for animals whose bodies are soft and not reinforced by a rigid skeleton. It is not a formal taxonomic group; rather, it encompasses diverse lineages united by the absence of a hard internal or external frame and a body that can be easily deformed. Many soft-bodied animals rely on hydrostatic pressure and flexible tissues to provide support and enable movement.

Morphology and physiology commonly associated with pehmeälihisiä include a soft, fleshy body, extensive musculature, and, in many species, a body cavity filled with fluid that works together with muscles to create a hydrostatic skeleton. While the term emphasizes softness, some organisms in this broad category may possess protective or supportive structures such as shells, calcareous plates, or spicules, which do not constitute a full rigid skeleton.

The group includes a wide range of phyla, such as Mollusca (for example snails and octopuses), Annelida (earthworms and leeches), Platyhelminthes (flatworms), Nemertea (ribbon worms), and Cnidaria (jellyfish and sea anemones). These organisms inhabit marine, freshwater, and terrestrial environments and exhibit varied life histories, feeding strategies, and reproductive modes, including sexual reproduction, asexual reproduction, and hermaphroditism.

Fossil records of pehmeälihisiä are often fragmentary due to the soft nature of their tissues, though exceptional preservation in Lagerstätten provides insights into their early evolution. The concept remains useful for comparative anatomy, developmental biology, and discussions of animal form and ecology.