Final answer:
Cnidaria and Ctenophora have been placed into separate phyla due to differences in cell types, symmetry, embryologic development, and genetic makeup, supported by molecular studies indicating a separate evolution of nerve and muscle cells.
Step-by-step explanation:
Cnidaria and Ctenophora have been placed into separate phyla because of significant differences in their morphology and genetics. The phylum Cnidaria includes animals with specialized cells known as cnidocytes which contain nematocysts, and they exhibit either radial or biradial symmetry.
They are diploblastic and develop from two embryonic layers. On the other hand, Ctenophora, also known as comb jellies, do not possess cnidocytes and have a different set of features, such as the absence of Hox genes and a much smaller mitochondrial genome.
Furthermore, molecular studies suggest that their nerve and muscle cells evolved independently from those in Cnidaria.