Now the Lord had prepared a great fish to swallow up Jonah. And Jonah was in the belly of the fish three days and three nights.:
Jonah 1:17, KJV
Now that we have met some of the strangest members of the Kingdom of Animals, we are ready to move on to the last group of animals – those with backbones, known as vertebrates. Your backbone helps to give your body structure and to connect your brain more securely with the rest of your body. All animals with backbones, from tadpoles to Tasmanian devils, are in one group, and they include all fish, amphibians, reptiles, birds, and mammals (it’s ok if you don’t know what all of those are just yet – we’ll get there).
So far in the Kingdom of Animals, we have met many creatures who live on land, such insects, but we are going to head back into the shining oceans and babbling brooks to meet our first group of vertebrates: the fish. You have likely seen many fish in your life, maybe in an aquarium or a pet store or even your own living room, so you know that fish is an animal that lives in the water and usually has a tail, gills (slits on their sides that they breathe through), and several fins (rigid or move-able fan-shaped “arms”) to help them move through the water. But not everything living in the water is a fish. How can you tell them apart? It can be quite confusing, especially when many creatures that are not true fish have the word “fish” right in their name, such as starfish, jellyfish, crayfish, and cuttlefish; scientists found this so problematic that they actual starting calling some of these creatures by different names, such as sea star and sea jelly. If you were visiting the ocean and saw these creatures, though, you would be unlikely to confuse the ephemeral jellyfish or the squid-like cuttlefish with a tuna or shark: they simply look too different with their lack of tail, fins, and general “fish” shape.
There are, however, some fish which are difficult on first glance to be sure whether they are really fish at all. Consider the sea horse: this little guy floats through the water without a tail or hardly any fins. Can this creature be a fish? Believe it or not, sea horses are fish since they have a backbone; their tail is still there, just made for holding onto seaweed rather than rushing through the water. How about a stingray or a hagfish (a creature that looks a bit like a headless snake)? These creatures seem like they don’t have bones – can they be fish? Yes! These are also fish – not all fish have bones, some have skeletons made of cartilage (the same thing that your nose and ears are made of), which helps them to maneuver through the water.
Fish come in all shapes and sizes, but another thing they all have in common, as do amphibians and reptiles, is that they are cold-blooded. Wait, fish have cold blood? No! Cold-blooded just means that their bodies do not stay the same temperature all the time like yours does but rather changes according to the temperature around them. This is why you will see lizards or crocodiles sitting on rocks in the sun – they are trying to warm themselves up. (There are actually a few kinds of fish that are not completely cold-blooded, including tuna and some kinds of sharks; they have been given the ability to keep themselves a bit warmer than the ocean around them to help them catch their prey more easily).
While fish live their whole lives in water, amphibians live only part of their lives in water. Amphibians include such common animals as frogs, toads, and salamanders, as well as some surprising ones like the axolotl [ax-oh-lot-ul] (with its fluffy external gills) and a caecilian ([si-sill-ian] a cross between a snake and worm that lives underground). Amphibians usually lay their eggs in water, since their eggs don’t have a shell like the chicken eggs that you had for breakfast; rather, they look like tiny black and clear pearls with tiny creatures (sometimes) visibly growing inside them. Once they hatch, young amphibians live in the water for a time, changing in shape as they grow, from a fish-like creature into its adult form with four legs (we’ll talk more about this miraculous change in the future).
As adults, amphibians usually have smooth, wet skin (although toads and newts tend to have drier, warty skin). Their skin gives amphibians a special super power: they can breathe underwater through their skin or in the open air through lungs. Because their skin needs to stay moist (a little wet), nearly all amphibians live near water and move easily in and out of the ponds, lakes, and streams that they frequent.