To help us understand the differences between the animals in our world, scientists classify, or sort, animals into different groups that share important characteristics. The two largest groups of animals are vertebrates and invertebrates.
Vertebrates are animals that have spines, or backbones. Invertebrates are animals that do not have backbones. Mostly animals are invertebrates. Some examples of invertebrates are octopus and squid, insects, arachnids, starfish and sea urchins, jellyfish, crabs and lobsters, clams, and many more creatures.
On land most invertebrates are fairly small, but in the water they may grow to much larger sizes, as the water will support them even without a skeleton. Vertebrates make up less than five percent of all animal species on earth, but if I asked you to think of an animal you would probably choose a vertebrate!
Vertebrates are divided into different classes, or groups. Several of these classes are specific types of fish, for example: jawless fish, like lampreys and hagfish, and cartilaginous fish, like sharks and rays.
For simplicity’s sake, these other classes are usually lumped together with bony fish and just called ‘fish’.
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