Chapter 6B: The Kingdom of Animals - Invertebrates

Part 2: What Is an Animal? Invertebrates Show Us the Way

All things bright and beautiful,
All creatures great and small,
All things wise and wonderful,
The Lord God made them all.

-Cecil Frances Alexander

Before we set out in earnest to explore the wilds of the Kingdom of Animals, we ought to consult its atlas. This one is thick – with 31 different regions (phyla), ranging from the 1.25 million members of Arthropoda (those would be the insects, crustaceans like crabs and lobsters, arachnids like spiders and scorpions, and myriapods like centipedes and millipedes) to the singular member of Micrognathozoa (the Limnognathia that we mentioned earlier). All of the vertebrates are in just one phylum, and the other 30 phyla show the rich diversity in those that are spineless.

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 human!) 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 in their bodies, while protists never do (in fact, protists are almost exclusively unicellular, too). 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: a sponge can move parts of its body to control the water that brings it food, but 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. (Whew! That’s a long list, thanks a lot sponges!)

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 to breathe.

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 so they don’t actually need to drink.

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 for some creatures it might be their own shell). Being a pet, she also need extra care and shelter compared to wild animals, which is why domesticated animals of all types are often given housing by people.

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 and dolphins. You won’t find a beaver doing bench presses or a tree frog on the treadmill, though, since they get any exercise that they need as they go about their lives.

Does an octopus need fresh air? Animals that live on land need fresh air to survive, but many animals live in water and have been given the ability to get the oxygen they need from out of the water itself. The Creator has given to each creature the home that it needs to care for its needs, so animals can care for themselves most admirably.

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