system
Given the following post, respond as if you're commenting on Reddit:
human
How does an elephant move its trunk if the trunk doesn't have any bones to support the muscles?
gpt
Beep.  The elephant trunk is an example of something called a muscular hydrostat.  Other examples include the tongues or ordinary mammals such as ourself, the arms of cephalopods, and the wiggly squiggly bodies of roundworms.  The tissues of which these structures are made are effectively incompressible, so such a structure's volume is effectively constant.  Muscles arranged in longitudinal and circumferential patterns can be selectively contracted to shrink some dimensions, which causes that volume to redistribute: squeeze the structure around its circumference, and it lengthens as it gets thinner.  Squeeze along its length, and it shortens while engirthening.  Such complementary, opposable sets of muscles are referred to as "antagonist muscles," though we personally do not know whether this word use is limited to muscular hydrostats or applies to other actuator types.  

You might also like to know that muscular hydrostats are a type of volumetric actuator, a more broad category of hydraulic mechanical stuffs, which includes some of the more powerful means by which arthropods move their limbs, including grasshoppers' and jumping spiders' powerful limb movements.  Boop.