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There are two groups of bilaterally symmetrical animals: protostomes and deuterostomes. Protostomes contain arthropods, molluscs, and more, while deuterostomes include echinoderms and vertebrates. Protostomes are divided into three superphyla: Lophotrochozoa, Edcysozoa, and Platyzoa. Lophotrochozoans have a lophophore and include marine animals, while platyzoans are flat animals with a diminished coelom. Ecdysozoans molt their exoskeletons and include arthropods and nematodes, the most numerous animals on Earth. Arthropods contain the most species and account for over 90% of all animal biodiversity.
There are two main groups of bilateral (bilaterally symmetrical) animals: protostomes and deuterostomes. They are also called superphyla, because each contains multiple animal phyla. Deuterostomes tend to be more complex and include echinoderms, hemicordates, chordates (all vertebrates), and some smaller phyla. Protostomes contain just about everything else, including arthropods, annelids, flatworms, molluscs, etc. Some small phyla are not classified as either. The main difference between the two groups is how they develop as embryos: in protostomes, the first opening in the embryo becomes the mouth, and in deuterostomes, it becomes the anus.
Most scientists recognize at least three superfilas within protostomes: Lophotrochozoa, Edcysozoa and Platyzoa. These superphyla were proposed in 1995, 1997 and 1998 respectively, based on molecular and morphological evidence. The lophotrocozoa (“crest-bearing animals”) are united by the presence of a lophophore, a ring of ciliated tentacles around the mouth, used to grasp 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 mouth tentacles for grasping food are more useful underwater.
Besides the lophotrochozoans, another predominantly marine superphyla of protostomes are the platyzoans (“flat animals”), which include flatworms and a number of important microscopic and planktonic animals such as rotifers and gastrotrichans. Platyzoans are distinguished by either the absence of a coelom (body cavity) or a diminished coelom called a pseudocoele. On average, the platyzoans are the simplest of the superphyla protostomes, although simple organisms are also found among the lophotrocozoans. Platyzoans were previously considered members of Lophotrochozoa, but most scientists now give them their own group.
The last superphyla protostomes are the Ecdysozoa, characterized by the moult of their exoskeletons (ecdysis). Ecdysozoans are the most widespread and successful protostome superphyla and include 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 imaginable environment, including Antarctica, while arthropods contain the most species, probably over 7 million, accounting for more than 90% of all animal biodiversity on Earth . While there are numerous other organisms that use some form of shell or external cuticle, the Ecdysozoa are obviously the most evolutionarily successful.
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