How does a sea anemone go to the bathroom?

It doesnt

Not true. Octopuses are a type of cephalopod. They have a full digestive system. They have an anus and eliminate waste through the siphon.

Here's a link to information on cephalopods:

http://www.britannica.com/EBchecked/topic/103036/cephalopod/35765/Form-and-function