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

Metazoa

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. Their life cycle invariably includes 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 colonial flagellates. It is hypothesized that individual cells within such colonies underwent functional differentiation related to locomotion, feeding, reproduction, and other tasks. Subsequently, internal and external Cell layers began to develop in these colonies, and gradually, through millions of years of evolution, the lower multicellular animals emerged.

The subregnum of multicellular animals comprises 9 phyla: Sponges (Spongia), Coelenterates or Cnidarians (Coelenterata seu Cnidaria), Flatworms (Plathelminthes), Roundworms (Nemathelminthes), Segmented Worms or Annelids (Annelida), Arthropods (Arthropoda), Mollusks (Mollusca), Echinoderms (Echinodermata), and Chordates (Chordata). This study guide focuses in detail exclusively on invertebrate phyla.



Last update: 13/08/2026

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