Traditional classication of Bilateria bases upon the idea of the progressive evolution from simplicity to complexity. Parenchymatous animals (e. g., Plathelminthes) were considered to be the most primitive. Pseudocoelomate worms (Nemathelminthes, Rotifers, etc.) were thought to be the second level of complexity, while the coelomate animals were believed to be the most advanced metazoans. The succession parenchymatous worms – pseudocoelomate worms – coelomate animals was the basis of zoological education in high school as well as in universities all over the world. Strict consistency of this system made us to turn a blind eye to some obvious defects of the traditional classification. It was well known that plathelminths had extremely complicated genital system, internal fertilization, extraordinary modified spermatozoa, and nobody was able to produce satisfactory explanation how external fertilization and primitive spermatozoa of coelomates could be derived from the plathelminth’s conditions. Plathelminths have determined spiral cleavage of egg (or even more modified clevage) that makes impossible to derive the primitive radial cleavage of lophophorates and deuterostomians from the plathelminths type of development. There is no satisfactory explanation why arthropods which were considered to be the closest relatives to annelids have neither spiral cleavage nor trochophore larvae. The list of contradictions of the classical system could be extended.
New discoveries in the fields of molecular phylogeny, developmental biology, and paleontology performed a radical change in views on the phylogeny and classification of bilaterians. According to new classification, based upon the molecular phylogeny, bilaterians are subdivided into 4 principal groups: Deuterostomia, Ecdysozoa, Lophophorata, and Trochozoa. The new system differs from the traditional view in some essential aspects. New system ruins the unity of Articulata which held out in zoology for about 200 years. New classification denies the kinship between Annelida and Arthropoda. In the new system, articulates and round worms are drawn together instead and the new high level taxon Ecdysozoa arises. Lophophorates were traditionally considered to be close related to deuterostomians, but the new classification insists on the uniting lophophorates and trochozoans into the supertaxon Lophoptrochozoa.
Within the framework of new bilaterians phylogeny the ancestral bilaterians are assumed to be having a relatively complex organization e.g. tube like gut, coelom and metameric limbs. Acoela and typical plathelminthes are supposed to be the progenetic groups retaining the larval organization of extinct coelomate ancestors. Acceptance of complex organization last common bilaterian ancestor let us find the explanation of so called “Cambrian explosion” that means the almost synchronous appearance in early Cambrian time morphologically complicated animals (arthropods, lophophorates, echinoderms and even chordates) which occupy the highest levels in classical system. In nonclassical zoology, many traditional problems turn out “head over heels”. Now, one has to study not why and how the coelom originates but why and how it disappears in some cases, not why and how the metamery arises, but why and how it was lost, not why and how limbs form in evolution, but why and how they reduce, etc. The new events in zoology promoted by molecular phylogeny could be considered as one of scientific revolutions which break gradual development of science from time to time.