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...

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

That's beautiful