Chondrichthyes

1) They're a diverse group containing more than 700 species  

2) They all lack the air filled swim bladder , therefore they have to always swim in order to stay afloat.

3) The Chondrichthyes are considered the oldest existing jawed-vertebrates 

4) These fishes have teeth-like scales, denticles, which include the sharks, rays, skates and ratfishes.

5) Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, scales, a heart with its chambers in series, and skeletonsmade of cartilage instead of bone.