What Is a Hydrostatic Skeleton?


If you’re reading this, then chances are pretty good that you have a skeleton. When most of us think of skeletons, we think of the bones in our body holding us up, keeping us from deflating into a flat mess of flesh. But there’s another, lesser known type of skeleton that, believe it or not, doesn’t involve bones at all.


Not all animals have a vertebral column (spine) and/or any other type of solid bone structure. In fact, 95 percent of all animals share this trait. Known as invertebrates, they are the most diverse group of creatures on the planet, which invariably means that they inhabit the most diverse landscapes on the planet. From the bottom of oceans to the thick of jungles, many invertebrates use special biological structures, known as hydrostatic skeletons, to make their lives possible.


Instead of bones, hydrostatic skeletons use a coelom, or a cavity filled with organic fluid, encased by muscles. The combination of rigidity created from the fluid and the movement of the muscles together serve as a support structure for the organism, while also allowing for locomotion. Imagine a long balloon; when it’s empty, it’s flat, but fill it with water and the surface will become rigid and solid. The same principle applies, particularly since the fluid filling the coelom is primarily made up of water, hence “hydro” in the name “hydrostatic skeleton.”


Hydrostatic skeletons are useful for locomotion because of their flexibility. Earthworms, which are completely boneless, use their skeletons to burrow through the ground. Jellyfish use them to open and close their heads (known as bells) and promote drifting.


Some notable animals that make use of hydrostatic skeletons are, as previously mentioned, earthworms and jellyfish. Other organisms include echinoderms, which are among the most common groups of marine animals, with the phylum encompassing everything from starfish to sea urchins. Nematodes, possibly one of the most diverse of all animals with over 80,000 described species, have a body structure that is almost completely made up of a single hydrostatic skeleton.


Though human beings are vertebrates, which make up roughly 5 percent of the animal population, we do have a structure very similar to, but not quite the same, as a hydrostatic skeleton: Our tongue. The human tongue, in fact most animal tongues, are known as muscular hydrostats, which are very similar to hydrostatic skeletons in that they are inoperable without fluid (usually water). Muscular hydrostats, however, lack a coelom, and instead are made completely out of muscle. Other animals and their similar structures include elephant trunks, octopus tentacles and mollusk feet.

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