Question: Have you found a species of animal that some of that species has a backbone and others don't?

Yes, the sea squirt (Ascidiacea). In the adult there is no backbone or spinal cord, but in the larvae stage there is!! The larvae of a sea squirt looks like a little fish, it has a simple backbone called a Notochord. When the larvae settles it lands with its head on a rock and a type of glue is secreted to stick it there. Then it changes into an adult, which looks like a big blob and the notochord is reduced to a very simple nervous system.

Search for sea squirt on Google to see some photographs.