All things bright and beautiful, All creatures great and small, All things wise and wonderful, The Lord God made them all.”
Cecil Frances Alexander
Now that we have gotten to know some of the fantastic members of the Kingdom of Animals, let us now consider its atlas. There are a shocking 31 different regions or phyla in this kingdom, giving it the most recognized diversity of any of the kingdoms. Even more surprising, all of the vertebrates (animals with backbones like us) are in just one phylum. The rest of the 30 phyla are composed entirely of the invertebrates that we have been describing here. Some of these phyla are astonishingly small, such as the phylum Micrognathozoa, home to just one species, the Limnognathia that we mentioned earlier. Can you imagine that, a whole huge region for just one small creature that grows no larger than the thickness of a human hair? At least it will have plenty of elbow room. The members of Arthropoda, on the other hand, might get a bit cramped, with 1.25 million species of insects, crustaceans (like crabs and lobsters), arachnids (the spiders and scorpions), and myriapods (the creepy-crawly centipedes and millipedes). Watch where you step!
Most creatures are easy to classify, after all no one would mistake a trout for a tree or a blue whale for a bacteria. When we come invertebrates, however, it can become much more difficult to differentiate among the kingdoms. Consider, for example, a sponge – an immobile sea creature shaped like a vase that might wave gently in the current but otherwise just sort of sits there, looking for all of the world like a plant. And that is exactly where scientist put this creature for many years, but no longer! Our sweet little invertebrate (ok, maybe not so little – some are bigger than a person!) is now home where it belongs in the Kingdom of Animals.
But why? What makes an animal, an animal? A sponge is fairy obviously not a bacteria or archaea, after all it is multicellular (has more than one cell) in addition to being a eukaryote. (You remember what that is, right? It has a nucleus in each of its cells.) Perhaps it ought to be a protist? No dice, sponges have different types of cells, while protists never do (in fact, protists are almost exclusively unicellular, too). Animal-like protists are heterotrophic (get their food rather than make it), ljust ike sponges and other animals, but they generally lack the complexity that we see in true animals. Despite the green-ish appearance of some sponges, they are not autotrophic like plants. What about a fungus? Fungi are, after all, heterotrophic, multicellular, and have different types of cells, just like sponges. The difference comes in the sponge’s ability to move: while a sponge can move parts of its body to control the water that brings it food, a fungus cannot move at all. So, that is what makes an animal, an animal: they are multicellular, heterotrophic creatures that have different kinds of cells and can move around at least part of their bodies.
What do animals need to survive and thrive? Well, what kinds of things do you need every day? You need food, of course, with carbohydrates, protein, fats, and vitamins and minerals. You need to drink water and get plenty of rest, as well as a safe place in which to get it. You also need exercise to keep your body strong and fresh air with oxygen.
What about other animals, do they need the same things? Does a frog need food and water? Of course – all animals need to eat food (being heterotrophs and all) and water of some kind, although many animals live in salt water, which would make us sick if we drank it. Does a dog need rest and shelter? My dog is currently sleeping by my feet, so I’m going to guess that they need rest. Dogs, like all animals, also need to have somewhere safe to go (although it might be their own shell), but being pets, they need extra care and shelter compared to wild animals. Does an otter need exercise? All animals move at least a little to get their food, and some animals even play like we do, including otters! Does an octopus need fresh air? Of course not – many animals need to live in water because they have been given the ability to get the oxygen that their bodies need from out of the water. Aquatic species (those that live in water) are so common that every animal phylum has at least one such species except for velvet worms (flat caterpillar-like creatures that indeed look like they are covered with velvet).