Here's why:
* Mollusks are invertebrates, meaning they lack a backbone.
* Reptiles, mammals, amphibians, and fish are all vertebrates, meaning they have a backbone.
This fundamental difference in body structure places mollusks in a completely different branch of the animal kingdom.
Here's a simplified breakdown:
* Kingdom: Animalia
* Phylum: Chordata (Vertebrates)
* Class: Mammalia, Reptilia, Amphibia, Pisces (Fish)
* Phylum: Mollusca (Mollusks)
While all these animals share a common ancestor, they diverged millions of years ago, developing distinct evolutionary paths and body plans.