What is an animal (Opisthokonta: Metazoa)?

Sponges, cnidarians, annelids, vertebrates, arthropods, echinoderms, molluscs, and a large diversity of obscure “worms” and tiny meiofaunal organisms make up the Metazoa Haeckel 1874, the group more commonly known as “animals”. But with the enormous diversity of unicellular and multicellular eukaryotes, what are the features that set animals apart from the rest of biology? If you extract some strange, never-before-seen organism from between marine sand grains, how can you determine whether or not this is an animal or just a colonial clump of some unicellular eukaryote?

There are several metazoan autapomorphies, unique characteristics that set animals apart and that are considered to be in the ground plan of the animals:

  • Multicellular organism.
  • Tight junctions between cells.
  • An extracellular matrix containing collagen.
  • Oogensis, spermiogenesis.
  • Radial cleavage leading to blastula formation.

All animals are truly multicellular, and several of their autapomorphies have to do with this trait.

Source: Wikipedia

Tight junctions, technical name zonulae occludens, are where the cell membranes of the multicellular animal meet. The membranes pseudo-fuse together using special protein complexes, thus stopping chemical exchange from the outside to beneath the cells (in the diagram: from the apical to the basolateral side). This set-up can then easily be extrapolated to tissues or organs – although note that true cell-cell interactions required for organ function are not a feature found at the base of the Metazoa (e.g. sponges have no organs or epithelia), so tight junctions are just an important prerequisite.

Source: Rozari & Simone
Source: Rozario & DeSimone (2010)

The other major feature related to multicellularity is the evolution of the extracellular matrix (ECM) – if you ever find that strange unknown organism from the opening paragraph, your first order of the day is to look for ECM, because if it’s there, you definitely have an animal on your hands. In most animals, the ECM is just a thin fluid layer between cells, although there are exceptions: the sponge mesohyl that makes up most of the sponge body, and the jellyfish mesoglea (“bell”), are both relatively huge cavities that are filled with ECM.

ECM is is a cocktail of water, fibrils (collagen, elastin, chitin), and glycoproteins (fibronectin, laminin). It’s produced by the cell and excreted into the body where it performs a variety of functions, as the diagram above notes. Its most important function in our context is that holds cells together, thereby making sure that the animal isn’t just a clump of aggregated cells, but a truly integrated organism.

Source: Wikipedia
Source: Wikipedia

I already discussed how a system consisting of sperm and egg theoretically evolves in this post. The animalian autapomorphy here isn’t quite the sperm and egg, but the processes behind their formation: oogenesis and spermiogenesis. Oogenesis, diagrammed above, leads to the formation of one egg and three degenerate, plasma-poor polar bodies. Spermiogenesis starts from one spermatocyte and finishes off with four fully-functioning sperms. Both of these are conserved across all animals, so it’s reasonable to conclude that they’re typical animalian features. However, do note that having gonads (penis, vagina, cloaca) is not necessary. Many animals, including basal ones, don’t have any, and just release their gametes freely into the ocean through a pre-existing water canal (e.g. in sponges) or pore (e.g. the urogenital pore of priapulids).


Haeckel E. 1874. Die Gastrea-Theorie, die phylogenetische Klassifikation des Tierreichs, und die Homologie der Keimblätter. Jenaische Zeitschrift für Naturwissenschaft 8, 1-55.

Rozario T & DeSimone DW. 2010. The extracellular matrix in development and morphogenesis: A dynamic view. Developmental Biology 341, 126-140.

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