Some still do: it's called a gastrovascular cavity, and is present in marine invertebrates such as Cnidaria (jellyfish, coral polyps, anemones), Ctenophores (co

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).