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Chapter 14 - From Protozoa to MetazoaAnimals made of multiple cells, metazoans, have a very long history on Earth. The periods of the pre-Cambrain (Ediacaran) and Cambrian are said to have experienced an “explosion” in geological terms. Many present-day phyla of animals appear for the first time in Ediacaran and Cambrian sediments and many other phyla are now extinct. One must remember that these periods of Earth’s history span nearly 150,000,000 years and that the phyla represented are characterized by their most ancient members. The origins of the metazoa have been sought not only through fossil evidence but also via studies of embryology and development.
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