Are there any mammals that don’t have a backbone?

Question: Are there any mammals that don't have a backbone?
- Keywords:
  animal,
  bone,
  mammal
Asked by gecko678 to Suzanne on 28 Jun 2013.
- Suzanne Harvey answered on 28 Jun 2013:
  
  Every mammal has a backbone, but not everything that has a backbone is a mammal – fish are a good example of this. Including non-mammalian animals with a backbone, they still only make up about 4% of all species!

Question: Are there any mammals that don't have a backbone?