The embryo, not the fetus. Pretty sure that's referring to gastrulation, where the cell blob splits into multiple layers to start cell specialization; it does this by curving in on itself, and the cavity formed by this process becomes the intestinal tract: https://en.wikipedia.org/wiki/Gastrulation#Protostomes_versu...
However, that all happens at the embryonic stage (around day 9). At the point where the embryo has advanced sufficiently to be called a fetus (around week 9), the body has already sufficiently developed so that it's impossible to say where "fetal development" starts: https://embryology.med.unsw.edu.au/embryology/index.php?titl...
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).
However, that all happens at the embryonic stage (around day 9). At the point where the embryo has advanced sufficiently to be called a fetus (around week 9), the body has already sufficiently developed so that it's impossible to say where "fetal development" starts: https://embryology.med.unsw.edu.au/embryology/index.php?titl...