Here's why:
* Vertebrates have a backbone (spinal column) made of vertebrae. This provides support and structure for the body. Opossums have a clearly defined backbone.
* Invertebrates lack a backbone and typically have an exoskeleton (external skeleton) for support.
Opossums also have other characteristics of vertebrates, such as:
* A skull containing a brain
* A well-developed nervous system
* A muscular system
* A circulatory system with a heart
* Internal organs like a stomach, intestines, and liver.