Diversity of living things
Classification sorts living things into groups by shared structures, and whether an animal has a backbone splits it into vertebrate or invertebrate before any finer grouping.
A worked example
A salmon has a backbone, so it is a vertebrate before any other detail is even considered. A jellyfish has no backbone, so it is an invertebrate, even though both live in water.
The mistake people actually make
Where an animal lives is not what classification is based on. A jellyfish and a salmon share the ocean but sit on opposite sides of the vertebrate/invertebrate split.