The sponge: the earliest metazoa. Image from Cruisecortez.com
Higher order taxa
Eukaryota; Fungi/Metazoa group
Description and Significance
Metazoan animals are multicellular, mitochondrial eukaryotes. Today Metazoa encompasses all animals with differentiated tissues, including nerves and muscles. They evolved from the protists approximately 700 years ago. There are two prominent theories dealing with how the metazoans came to be, although one, the syncytial theory, has been somewhat discredited. The other, the colonial theory proposed by Ernst Haeckel in 1874 states basically that multicellular organisms have a colonial ancestor. This is in keeping with the idea that the choanoflagellates, a group of colonial protists, created the colonies from which multicelled organisms first evolved. This evolution occured sometime during the Precambrian period; the oldest known animal fossils were discovered in Precambrian rocks in 1946.
Many genomes of metazoan animals have been sequenced; see NCBI's Genome category for complete results.
Cell Structure and Metabolism
Image from University of Manitoba.
Metazoan animals are heterotrophic; they consume other organisms or the products of other organisms because they cannot produce their own energy. Primarily they have diploid life cycles; that is, the male organism (sperm) fertilizes the female organism (egg), producing a multicelluar embryo which becomes a new diploid organism. A characteristic unique to the metazoic cell is the extracellular matrix, made up of collagen, proteoglycans, adhesive glycoproteins, and integrin. These molecules serve to fill the spaces between cells, and are necessary structurally and sometimes developmentally. Metazoic cells have no cell walls.
Metazoan animals are found worldwide, in all climates and habitats. Their primary ecological role is that of consumer, of which there are three types:
- predator: animal consumes other organisms or something another organism has produced; includes carnivores and herbivores.
- parasite: animal lives on or in and obtains nourishment from a living host. May or may not be particularly pathogenic.
- detrivore: animal feeds on dead or decaying organic matter. Plays an essential role in the food web as recyclers.