Sea cucumbers breathe through their butts.

Sea cucumbers have a special internal organ called a respiratory tree. This organ consists of two long, branching tubes connected to the cloaca, which is the animal's rear opening. To breathe, the sea cucumber uses muscles to pull water into the cloaca. This water is then forced into tiny tubules where oxygen enters the animal's body fluid.A sea cucumber like the Holothuria scabra pumps water 10 to 20 times every minute. This method is very efficient for animals living on the seafloor where oxygen levels can be low. The process is entirely separate from how fish use gills on the sides of their heads.This opening is also used for other things. Some small fish, like the Pearlfish, swim inside the sea cucumber's rear to hide from predators. Researchers at the University of Liège found that these fish wait for the sea cucumber to open its cloaca to breathe before darting inside.If a sea cucumber is attacked, it can shoot its internal organs out of its rear. This is called evisceration and it distracts the predator while the sea cucumber crawls away. The sea cucumber can regrow its respiratory trees and other organs in a few weeks.