What is the least evolved animal living today?

The term "least evolved" can be misleading, as evolution doesn't imply a linear progression towards complexity. However, some of the most ancient and "primitive" animals include species like the horseshoe crab and certain types of sponges. Horseshoe crabs have existed for around 450 million years with minimal changes, while sponges represent some of the earliest multicellular life forms. Both are often cited as examples of animals that retain characteristics of early life forms.