Millipedes are myriapods, a type of arthropod.
There are two main groups of bilateral (bilaterally symmetrical) animals: protostomes and deuterostomes. They are also called superphyla, because each contains several animal phyla. Deuterostomes tend to be more complex and include echinoderms, hemichordates, chordates (all vertebrates), and some smaller phyla. Protostomes contain practically everything else, including arthropods, annelids, flatworms, molluscs, etc. Some small phyla are classified as none. The main difference between the two groups is how they develop as embryos: in protostomes, the first opening of the embryo becomes the mouth, and in deuterostomes, it becomes the anus.
Symphylans and pauropods are myriapods.
Most scientists recognize at least three superphyla within the protostomes: Lophotrochozoa, Edcysozoa, and Platyzoa. These superphyla were proposed in 1995, 1997 and 1998, respectively, based on molecular and morphological evidence. Lophotrochozoans (“crested animals”) are united by the presence of a lophophore, a ring of ciliated tentacles around their mouth, used to grasp food particles. The lophotrochozoans, sometimes also called trochozoans, include about a dozen phyla: Entoprocta, Mollusca, Annelida, Phoronida, Brachiopoda, Nemertea, Chaetognatha, Bryozoa, Myzostomida, Acoela, and Gnathostomulida. Most lophotrochozoans are marine, which makes sense because the ciliated tentacles of the mouth for catching food are most useful underwater.
The edcisozoans include crustaceans.
In addition to the lophotrochozoans, another predominantly marine superphylum of protostomes are the platyzoans (“flat animals”), which include flatworms and several important microscopic and planktonic animals, such as rotifers and gastrotrichs. Platypuses are distinguished by the absence of a coelom (body cavity) or by a diminished coelom called a pseudocoel. On average, the platyzoa are the simplest of the protostomes superphyla, although simple organisms are also found among the lophotrochozoans. Platyzoans were previously considered members of the Lophotrochozoa, but most scientists now give them their own group.
The last protostomic superphyla are the ecdysozoans, distinguished by the shedding of their exoskeletons (ecdysis). Ecdysozoa is the most successful and widespread protostome superphylum and includes arthropods (crustaceans, insects, chelicerates, and myriapods), nematodes (roundworms), tardigrades, velvet worms, and several smaller phyla. Nematodes are one of the most numerous animals on Earth and are found in every conceivable environment, including Antarctica, while arthropods contain the majority of species, probably more than 7 million, representing more than 90% of all the animal biodiversity of the Earth. While there are several other organisms that use some sort of external shell or cuticle, Ecdysozoa is obviously the most effective from an evolutionary standpoint.