Muscular hydrostats.

In Harry Potter and the Chamber of Secrets, Professor Lockhart's inept attempt to heal Harry's broken arm with a spell instead removes all the bones in Harry's arm, leaving a useless floppy arm in its place. While a night of the potion Skele-Gro eventually regrows Harry's bones, this episode neatly highlights the critical role a rigid skeletal support structure plays in our ability to move. Without one, we are stuck. Yet some of the most dexterous and impressive examples of movement in the animal kingdom do so without such rigid support structure. Tongues, tentacles, and trunks, as memorably framed in a seminal paper by William Kier and Kathleen Smith, are in a unique class of appendages known as muscular hydrostats that include octopus arms, elephant trunks, squid tentacles, and many animal tongues (Figure 1). These appendages lack bones or other rigid support entirely and instead consist only of soft tissue such as muscle and connective tissue.