What formed first, you may ask, our mouths or our anus? It’s not completely unlike asking, “What came first, the chicken or the egg?” The answer to both questions is the latter: the egg, and, at least for us human beings, the anus.
We human beings are deuterostomes, which means “second mouth.” Think of the book in the bible, Deuteronomy, which means “second book” or “second law.” Protostomes are those creatures whose mouth formed first, before the anus. They include worms, insects, crustaceans, snails, squid, and octopi. Deuterostomes include us, other animals with backbones like lemurs, lizards, salamanders, frogs, and also the echinoderms, spiny skinned creatures like starfish, sea urchins, and sea cucumbers.
Does this mean that we are more closely related to starfish than to octopi? It seems insufficient to count only the beginnings, when we may have taken different paths but arrived at similar places: with two eyes, and the keen intelligence to open jars to get at what’s inside.
Another surprise is that when the first cells divide after fertilization, they are determined in protostomes, but not for deuterostomes. Is this way a starfish are able to keep regenerating arms? Do the cells maintain that undifferentiated ability to become whatever is needed? And how were people able to determine that the early cells in deuterostomes remained undifferentiated? Did researchers actually go in and separate some cells of a fertilized egg/blastula, and all the separated cells became whole animals?
Source:
https://www.mun.ca/biology/scarr/Protostomes_vs_Deuterostomes.html