Here's why:

* Lack of Jaws: Agnathans, like lampreys and hagfish, are characterized by the absence of jaws, a feature that distinguishes them from all other fish groups. This suggests a very ancient lineage.

* Skeleton: They have a cartilaginous skeleton, which is less complex than the bony skeletons found in more advanced fish groups.

* No Paired Fins: Agnathans lack paired fins (pectoral and pelvic fins), which are crucial for maneuverability and are present in most other fish.

* Ancient Origins: Fossil evidence suggests that Agnatha appeared in the early Cambrian period, making them one of the earliest vertebrate groups.

While Agnatha is considered the most primitive class of fish, it's important to note that the term "primitive" can be misleading. These fish have adapted and evolved over millions of years, developing unique features and survival strategies.