Which pair of characteristics would you use to place a fish in one the three classes agnatha cartilaginous fishes or bony fishes?

The two most important characteristics to use for classifying fish into the three classes Agnatha, Chondrichthyes (cartilaginous fishes), and Osteichthyes (bony fishes) are:

1. Skeleton Composition:

* Agnatha: Have a cartilaginous skeleton, meaning it's made of cartilage, not bone.

* Chondrichthyes: Also have a cartilaginous skeleton.

* Osteichthyes: Have a bony skeleton.

2. Presence/Absence of Jaws:

* Agnatha: Lack jaws entirely.

* Chondrichthyes: Possess jaws, and their teeth are modified scales.

* Osteichthyes: Possess jaws, with teeth that are distinct from scales.

In Summary:

* Fish with no jaws and a cartilaginous skeleton: Agnatha (e.g., lampreys, hagfish)

* Fish with jaws and a cartilaginous skeleton: Chondrichthyes (e.g., sharks, rays)

* Fish with jaws and a bony skeleton: Osteichthyes (e.g., tuna, salmon)