Fundamentals of Evolution - Korzh O.P. - 2006

Part III. PATHWAYS OF THE EVOLUTION OF LIFE

Chapter 20. Main Stages in the Evolution of Multicellular Animals

Multicellular animals are characterized by bodies composed of numerous Cells and their derivatives, which are always differentiated both structurally and functionally. As a result, The life cycle of Metazoa involves complex individual development—ontogeny—during which an adult Organism develops from a fertilized egg Cell (zygote). The ontogeny of multicellular animals involves the Cleavage of the egg into A large number of blastomeres and their sequential differentiation into germ layers and organ rudiments. The cells of all animals, except for tunicates (Urochordata), lack a cellulosic Cell wall. With few exceptions, animals grow only up to a certain age. A significant distinction between this group of living organisms and plants and Fungi is their individual activity, which shapes their behavior. Animal motility enables them to occupy a vast variety of ecological niches; therefore, their overall diversity and Abundance significantly surpass those of other eukaryotic groups.



Last update: 07/08/2026

Editorial and Educational Adaptation: This material has been compiled based on the primary/original source text. The project team performed an editorial review, corrected technical inaccuracies, structured sections, and adapted the content for an educational format.

What was processed:

  • elimination of formatting defects (OCR errors, structural breaks, corrupted characters);
  • editorial organization of content;
  • standardization of terminology in accordance with academic sources;
  • verification of factual statements against the original source text.

All mentions of the author, publication year, and origin of the primary text have been preserved in accordance with the source.