Vertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals

General characteristics

Multicellular Organisms are characterized by the presence of numerous Cells and their derivatives within the body. These cells are differentiated both structurally and functionally and, acting merely as parts of a complex Organism, have lost their independence.

In The life cycle of multicellular organisms, there is always a period of individual development (ontogeny) during which an adult organism develops from a fertilized egg, or occasionally from an unfertilized one (parthenogenesis).

Most biologists believe that Multicellular animals evolved from ancient unicellular flagellates that lived in colonies. It is hypothesized that within such colonies, individual cells underwent functional differentiation related to locomotion, feeding, reproduction, and other tasks. Subsequently, internal and external Cell layers began to develop within these colonies, and gradually, over millions of years of evolution, lower multicellular animals emerged.

The subkingdom of multicellular animals comprises nine phyla: Sponges (Spongia), Cnidarians (Coelenterata seu Cnidaria), Flatworms (Plathelminthes), Roundworms (Nemathelminthes), Segmented Worms, or Annelids (Annelida), Arthropods (Arthropoda), Mollusks (Mollusca), Echinoderms (Echinodermata), and Chordates (Chordata). In this tutorial, we will examine only the final phylum—Chordates—in detail.



Last update: 13/08/2026

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