Vertebrates have a backbone. But there are many organisms that have backbones, including fish, amphibians, reptiles, birds, and mammals! So, how do we distinguish between them? We put them in categories.
But sometimes it is difficult to put organisms into specific categories. Observe the above image. What kind of organism is it? A snake? A lizard? A newt? It looks like a snake because it does not have legs. But it is also has the head of a lizard. What other information do you need to know before you can classify the above organism? What questions do you need to ask?
You may ask: Does it have ears? What other organisms have similar hearts or lungs? Are the organism's genes more like those of lizards or to those of snakes? Do they live in water, on land, or both?
These are the type of questions scientists ask when they encounter an organism not easily classified. Consider the differences amongst organisms who are closely related as you read about vertebrate creatures. And then see if you can identify the above organism.
Image copyright Joel Bauchat Grant, 2014. www.shutterstock.com. Used under license from Shutterstock.com.