Don’t try to refer to non-chordates as a single class, would be my suggestion. Mark chordates (and tetrapods particularly) linguistically as the unusual case instead.
What measure of complexity of the brain’s organization wouldn’t show a big increase between invertebrates and humans?
with a little judicious editing, becomes:
What measure of brain complexity wouldn’t show a big increase in tetrapods, and humans in particular?
Don’t try to refer to non-chordates as a single class, would be my suggestion. Mark chordates (and tetrapods particularly) linguistically as the unusual case instead.
with a little judicious editing, becomes: