> the anus has no use without the mouth - but (in my naive understanding) it means that somewhere beyond this ancestor, certain groups of vertibrates split and

> the anus has no use without the mouth - but (in my naive understanding) it means that somewhere beyond this ancestor, certain groups of vertibrates split and went in opposite directions despite both being (presumably) decended from this thing. I understand the story differently: there were animals with a single opening. Then the animals with two openings evolved, but more than once, based on the animals we have today, there are at least two different evolution internal mechanisms to more than one opening: it's just how the cellular mechanisms work during the cell division (meaning the "programs" driving them are different). We can't say there were ever "anus-only" animals just as we can't say there were "mouth-only" animals. There were single-opening animals and then there were two internal mechanisms leading to the two-opening animals.