[throwaway173738]

I read somewhere that the first invertebrate animal ate and excreted through the same butthole-shaped oriface.

[rsa4046]

Some still do: it's called a gastrovascular cavity, and is present in marine invertebrates such as Cnidaria (jellyfish, coral polyps, anemones), Ctenophores (comb jellies); Platyhelminthes (flatworms, tapeworms).

[woadwarrior01]

Yeah, I learnt about this from the amazing Max Bennett book: A brief history of intelligence.