INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, OR BILATERIA

This section encompasses the overwhelming majority of Multicellular animals. Unlike radially symmetrical animals, bilaterally symmetrical organisms are characterized by bilateral Symmetry and triploblasty, meaning their bodies develop from three germ layers rather than two: the ectoderm, endoderm, and mesoderm. These animals exhibit distinct anterior and posterior ends, as well as dorsal and ventral sides. Only a single (sagittal) plane of symmetry can be passed through their body, dividing it into two mirror-image halves.

For centuries, the division of triploblastic animals into groups of related phyla sparked intense debate among taxonomists, even when classifications relied solely on morphological traits and embryological development. With the accumulation of molecular-genetic data and their comparison with morphological criteria, it became evident that traditional concepts regarding the origin and phylogenetic relationships of major animal groups required revision. Currently, several Classification schemes exist. According to the system proposed by V.V. Malakhov (2003), the section Triploblastica, or Bilateria, comprises five subdivisions: Spiralia, uniting 18 phyla; Ecdysozoa, comprising six phyla; Lophophorata, with three phyla; Deuterostomia, with three phyla; and Chaetognatha, represented by a single phylum of the same name.



Last update: 13/08/2026

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