As a body part, it is indeed the, uh, butt of many jokes. That said, there's no getting around the fact that it serves an important purpose. Our digestive system produces waste that cannot stay inside the body. A life without a butt to get rid of that waste? We don't want to think about how that would work out.
But (with one 't', stop giggling!) what if you had a disappearing butt? (Alright, you can laugh at that maybe...) One that could vanish when it was not in use?
Believe it or not, there is an animal that has this ability.
The sea walnut.
That's just nuts
Scientifically known as Mnemiopsis leidyi, the sea walnut (or warty comb jelly) is a species of jelly with an anus that appears only when needed. The anus is the official term for the body opening through which solid bodily waste leaves the body. Like our mouth, ears, or nose, it makes sense that this kind of opening would be permanent. So how exactly does the sea walnut's disappearing butt work?
Essentially, as waste begins to pile up inside the jelly's tummy, it starts to push against the outer tissue of the animal. This finally causes an opening to form, allow the waste to pass through. After the job is done, the opening closes right back up as though it never existed. Even under a microscope, no opening can be observed.
What does it all mean?
The evolution of the sea walnut can be traced back over 500 million years, making these animals something like living fossils. In a buttshell—*cough*, that should be nutshell, sorry—this curious creature could represent a time when primitive animals were first experimenting with the concept of a permanent butt.
What else would they have? you ask. (Are you sure that you want to ask that?)
Well, believe it or not, there are creatures with multiple anuses. (Funny!) And there are ones—like the medusa phase of jellyfish—with a single opening through which both food passes in and waste passes out. You see where this is going—their mouth is their butt. (Still glad that you asked?)
In all seriousness, the evolution of a permanent anus and a digestive tract was a huge biological event. And the sea walnut may be the kind of creature where that crucial change all began.
Which just proves the phrase: Every butt has a beginning. Wait, is that really a phrase? It is now!