If I not misheard (I don't know, just remember it), even the fetus development basically starts from the butthole.

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...

▲

throwaway173738 4 hours ago | parent | prev | next [-]

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

	▲	rsa4046 3 hours ago \| parent [-]
		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).

▲

chungusamongus 4 hours ago | parent | prev [-]

That's beautiful