sentence2_structure: Invertebrates (and higher animals<>can be placed<>$$$two groups<>based<>on how they develop as embryos$$$they<>develop<>
sentence1: Pycnogonids are one of the few groups of animals in which the males exclusive- ly brood the developing embryos and, in some species, the young.
sentence2: Invertebrates (and higher animals) can also be placed in one of two groups based on how they develop as embryos.
question: Invertebrates (and higher animals) can also be placed in one of two groups based on how they develop as what?