What is the Difference between Protozoans and Metazoans?

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In order to study the difference between protozoans and Metazoans, we should have a clear picture of what are protozoans and what are metazoans. So, the word Protozoans came from the Greek words “proto” meaning, “First” and “zoa” meaning “animals”. Protozoa are a diverse group of single cell eukaryotic organisms. A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Historically, Protozoa were defined as a single-cell with anima-like behavior, although plant-like unicellular organisms are classified as algae. The Metazoa or Animalia are a major group or kingdom of multi-cellular eukaryotic organisms. In general, they are capable of locomotion, responsive to their environment, and feed by consuming other organisms. Early on, Metazoa developed body layers, organs, a nervous system and finally a head, advanced sensory organs and a brain, leading to the development of intelligence. Both protozoans and metazoans have fuzzy ends.


What is Protozoa?

  • They are single celled organisms that are not an animal, plant, bacteria or fungi.
  • A single cell performs all functions; therefore there is no division of You do not have access to view this node.
  • They have a cellular grade of organization, e.g. Amoeba paramecium.
  • Protozoans are always motile, as there are no protozoans that can survive by stationary filter-feeding, in contrast to the numerous animals such as sponges, corals etc. Motility is a biological term which refers to the ability to move spontaneously and actively, consuming energy in the process. Filter feeding is a method of feeding where small food particles are filtered from the surrounding water by various mechanisms.
  • Many unicellular organisms also arrange themselves into a thread like or spherical colonies such as cyanobacteria (often referred as “blue-green algae) or cholorphyte volvox.
  • Protozoans range bigger from 10-50 microns (millionth of meter), just a few species are as large as 1mm.


What is Metazoa?

  • They are multi-cellular organisms.
  • They show division as different cells or organs perform different functions.
  • They have cellular You do not have access to view this node, organ and system grade of organization, e.g. Sponges to chordates.
  • Metazoans include about 50 microns, like the blue whale (34m or 111ft), lions mane jelly fish (36.5 m or 120 feet), and Bootlac worm (55m or 180ft). The extinct dinosaur “Amphicoelias fragillimus” might have been 60m (197 ft) or longer. So we see, the largest metazoans may be as much as a million times larger than a typical protozoan, though most metazoans are smaller in size, less than an ‘mm’ in length. Macroscopic metazoans such as humans are in minority.
  • It is though that metazoans evolved from protozoans approximately 600 million years ago, though molecular systematic evidence suggest even earlier. When the initial metazoan animals must have been really small, and derived from soft You do not have access to view this node, they would not have fossilized too well. Though one of the earliest known metazoan fossils, a small spherical animal called “Vermanimalcula guizhouena’, was a soft creature just 0.1 to 0.2 mm in size, barely large enough to see and about the width of a human hair.
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