Metazoa, are multicellular organisms generally recognized as animals. In the past, protozoa and metazoa were considered animals. In fact, metazoans reflected descent from a protozoan ancestor. Now, by animal we mean only the multicellular metazoa, known as Animalia. Metazoa are motile, heterotrophic, multicellular eukaryotes. Compared to protozoa, metazoans are large organisms.

Body structure

Metazoans have a body made up of specialized cells, which is a distinct difference from protozoa. In addition, the cells of metazoans are, not coincidentally, grouped into specialized “layers” called tissues. The major tissues that make up the body of a metazoan are

  • epithelial tissue
  • connective tissue

An epithelium consists of cells joined together to form a lamina that lines not only the body but also the internal cavities. Epithelial cells rest on a basal lamina, a fibrous extracellular matrix. Adhesion between epithelial cells is due to a “glue” that is nothing more than adhesion molecules.

Connective tissue is composed of cells that are not adherent as in epithelial tissue; in fact, the cells are separated by an extracellular matrix. The result is a less dense and compact tissue. The extracellular matrix contains collagen, which is not the only protein, but is certainly the most important.

The cellular matrix can be modified to form a skeleton, a fundamental structure because it provides support for the body. Metazoan skeletons can be:

  • Exoskeleton: composed of a thickened cuticle
  • Endoskeleton: stiffened extracellular matrix


Metazoans practice both sexual and clonal (asexual) reproduction.

As for clonal reproduction, it is achieved by

  • Fragmentation: the body fragments in an irregular manner.
  • Fission: it is an orderly division of the body
  • Gemination: differentiation of a child individual before separation from the parent body
  • Parthenogenesis: development of a child from an unfertilized egg or totipotent cell.

Sexual reproduction occurs through sperm and eggs. Both are formed by meiosis, and their fusion restores diploidy and produces a zygote with polarity. Polarity is due to the animal pole and the plant pole.

Origin of Metazoa

The origin of metazoans is currently the subject of much debate. Most zoologists believe that metazoans are descended from a common ancestor of even single-celled organisms.

According to the colonial theory, metazoans would descend from a colony of flagellated protozoa. This seems to be the most accepted hypothesis at present. The syncytial theory proposes that metazoans evolved from a plurinucleate but unicellular plasmodium.

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