Most organisms consist of only one cell and are invisible to the naked eye. Others, such as the rabbit, the bread mold and the pine tree, are made of many cells. Learn how to classify the multi-cellular organisms that inhabit our world.
Domain Classification
The living world is filled with a dizzying array of living things. Most of them are unseen, visible only under a microscope. These tiny organisms are unicellular, composed of only a single cell. The familiar plants, animals and fungi that we can see represent only a tiny fraction of life on Earth. These organisms, being made of more than one cell, are called multicellular.
Biologists classify organisms by common features, particularly DNA and other aspects of biochemistry. The largest recognized category of living things is the domain. These domains are subdivided into Kingdoms, Phyla, and so on, down to species, which is given in lower case.
Domain --> Kingdom --> Phylum --> Class --> Order --> Family --> Genus --> species
Life on Earth is currently divided into three domains. The evolutionary relationships in this diagram were determined by comparing ribosomal RNA. The kingdoms within each domain are also shown. Note that under Eukaryota, all of the kingdoms besides Animalia, Plantae and Fungi are often classified together as 'Protists.'
The Three Domains of Life
The first two domains are Archaea, whose members are called Archaeans, and Bacteria, whose members are called Bacteria. Archaeans and Bacteria are all unicellular. Members of both domains are called prokaryotes. Prokaryotes, in addition to being unicellular, lack a nucleus and other organelles.
Bacteria are prokaryotes.
This is a typical prokaryotic cell:
Prokaryotic Cell
A typical prokaryotic cell
The third domain is Eukaryota, whose members are called eukaryotes. Many eukaryotes are unicellular, and many are multicellular. Whether consisting of one cell or many, all Eukaryotic cells are complex, having a nucleus and other complex organelles. They also tend to be larger than prokaryotic cells.
Here is a typical animal cell:
Animal Cell
Eukaryote
Here is a typical plant cell:
Multicellular Kingdoms: Plants and Animals
This diagram above shows ten kingdoms under Domain Eukaryota.
Plants, animals and fungi are generally considered separate kingdoms by all scientists. Then, most scientists lump all of the remaining kingdoms into a single super-kingdom called 'protists.' We will look at each kingdom in turn, treating protists as one kingdom.
Animals are multicellular heterotrophs that lack cell walls and are generally capable of moving around. Heterotrophs do not make their own food, meaning they must obtain it elsewhere by eating other organisms or by finding dead material to eat.
Animal cells are specialized, which means they perform specific functions in the organism. For example, there are cells in the human eye that exist only to capture light. In an earthworm, there are muscle cells that are only concerned with movement. Some animals, like rotifers and tardigrades, are microscopic and are made of only a few hundred or a few thousand cells.
Rotifer
Rotifer
Others, like elephants, whales and people, are much larger. Most animals, such as insects, spiders and earthworms, fall somewhere in between. Animals are divided into various phyla, which are major groupings of similar organisms. You can see these groupings here: