The article about gastrulation from the other day was dreadfully vertebrate-centric, so let me correct that with a little addendum that mentions a few invertebrate patterns of gastrulation—and you’ll see that the story hasn’t changed.
Remember, this is the definition of gastrulation that I explained with some vertebrate examples:
The process in animal embryos in which endoderm and mesoderm move from the outer surface of the embryo to the inside, where they give rise to internal organs.
I described frogs and birds and mammals the other day, so lets take a look at sea urchins and fruit flies.