Most of the time, the small jellyfishlike creatures know as hydras don’t have mouths as we know them. Instead, when they need to engulf prey, the skin between their tentacles splits open and stretches into a maw that can grow wider than their bodies. Now, researchers have described the mechanics of this process for the first time, thanks to genetically modified hydras and new imaging technology that reveal the boundaries between different types of tissues. According to a study published online today in Biophysical Journal, hydras’ cells don’t rearrange to accommodate their expanding mouths; instead, each cell stretches into a dramatically deformed shape, as in the video above. Radial bands of musclelike tissue force the mouth open in a process that resembles pupil dilation in a human eye. When the researchers gave the hydras muscle relaxants to prevent these tissues from contracting, the creatures could no longer open their mouths, and the skin between their tentacles remained unbroken.