One kind of fish, the hagfish, is considered an invertebrate by scientists. Why is this fish an invertebrate whereas all other fish are vertebrates?

A hagfish does not have jaws and pectoral girdle or paired pectoral fins. They have poorly developed eyes and they only have one semicircular canals. They also don't have epiphyses. Hagfishes are considered to belong to the invertebrate group because they don't have spines.