Some jellyfish have only one orifice for the gut, which it uses for both ingesting food and expelling waste. The comb jellies, however, have a tube all the way through, like higher animals. However, the warty comb jelly (Mnemiopsis leidyi) has a different plan. Its digestive system goes in only one direction, but it stops at the end. That is, until the animal needs to poop. The anus forms only then, and when it's done its business, it closes up again. Sidney Tamm of the Marine Biological Laboratory in Woods Hole, Massachusetts, discovered the jelly's transient anus.
“It is not visible when the animal is not pooping,” says Tamm. “There’s no trace under the microscope. It’s invisible to me.”
His observations show there is no permanent connection between the gut and the rear of the body. Instead, as waste accumulates, part of the gut starts to balloon out until it touches the outer layer, or epidermis.
The gut then fuses with the epidermis, forming an anal opening. Once excretion is complete, the process is reversed and the anus vanishes.
Because both the gut and epidermis are just a single cell layer thick, this can happen relatively easily and quickly. The animals defecate at regular intervals: once an hour in the 5-centimetre-long adults, and once every 10 minutes or so in the larvae.