Pyukumuku’s “eyes” are never actually shown to blink or emote (when even Butterfree’s are, despite it having compound eyes). So… yup. That’s an anus.
I mean, the distinctly un-mouth-like shape and the parallels with real sea cucumbers did give me pause, but I was willing to give Pyukumuku the benefit of the doubt. Although… what benefit there is in vomiting up your lungs as opposed to farting them out, I’m not entirely certain. In any case, that’s the last time I trust a sea cucumber.