Millipedes are myriapods, a type of arthropod.
There are two primary groupings 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 a few smaller phyla. Protostomes contain virtually everything else, including arthropods, annelids, flatworms, molluscs, etc. Some small phyla are classified as neither. 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 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 grab food particles. 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.
Edcisozoa include crustaceans.
In addition to the lophotrochozoans, another predominantly marine superphylum of protostomes are the platyzoans (“flat animals”), which include flat worms and a number of important microscopic and planktonic animals such as rotifers and gastrotrichs. Platyzoans are distinguished by the absence of a coelom (body cavity) or by a diminished coelom called a pseudocoel. On average, the platyzoa is the simplest of the protostome superphyla, although simple organisms are also found among the lophotrochozoans. Platyzoans were formerly considered members of Lophotrochozoa, but most scientists now give them their own group.
The last protostomic superphyla are the Ecdysozoans, which are distinguished by the molt of their exoskeletons (ecdysis). Ecdysozoa is the most successful and widespread protostome superphyla 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, found in every conceivable environment, including Antarctica, while arthropods contain the most species, probably over 7 million, representing over 90% of all animal biodiversity on Earth. While there are several other organisms that use some form of shell or external cuticle, Ecdysozoa are obviously the most effective from an evolutionary point of view.