Animals

The Rules

Animals come in many different shapes and sizes, humans are a type of animal, mosquitoes are animals, rats are animals, spiders are animals, etc. Let's talk about the rules that separate animals from other living organisms.
(“The 8 Traits That Make an Animal an Animal”)

Rule 1 (Animals Are Multicellular):

Animals have multiple cells unlike bacteria and protozoa. Plants and fungi can also have multiple cells. There are other characteristics that set them apart.

(“The 8 Traits That Make an Animal an Animal”)

Rule 2 (Eukaryotic)

Possibly the most important split in the history of life on earth is the one between prokaryotic and eukaryotic cells. Prokaryotic organisms lack membrane-bounded nuclei and other organelles, and are exclusively single-celled; for example, all bacteria are prokaryotes. Eukaryotic cells, by contrast, have well-defined nuclei and internal organelles (such as mitochondria), and are capable of grouping together to form multicellular organisms. While all animals are euakaryotes, not all eukaryotes are animals: this hugely diverse family also includes plants, fungi, and the tiny marine proto-animals known as protists.

(“The 8 Traits That Make an Animal an Animal”)

Rule 3 (Specialization)

One of the most remarkable things about animals is how specialized their cells are. As these organisms develop, what seems to be plain-vanilla "stem cells" diversify into four broad biological categories: nervous tissues, connective tissues, muscle tissues, and epithelial tissues (which line the organs and blood vessels). More advanced organisms display even more specific levels of differentiation; the various organs of your body, for example, are made up of liver cells, pancreatic cells, and dozens of other varieties. (The exceptions that prove the rule here are sponges, which are technically animals but have virtually no differentiated cells.)

(“The 8 Traits That Make an Animal an Animal”)

Rule 4 (Moveability)

Fish swim, birds fly, wolves run, snails slide, and snakes slither--all animals are capable of movement at some stage in their life cycles, an evolutionary innovation that allows these organisms to more easily conquer new ecological niches, pursue prey, and evade predators. (Yes, some animals, like sponges and corals, are virtually immobile once they're fully grown, but their larvae are capable of movement before they become rooted to the sea floor.) This is one of the key traits that distinguishes animals from plants and fungi, if you ignore relatively rare outliers like venus flytraps and fast-growing bamboo trees.

(“The 8 Traits That Make an Animal an Animal”)