CLASS: Chondrichthyes—Cartilaginous Fishes, including chimeras, sharks & rays

(Chondrichthyes comes from Greek words meaning "cartilaginous fishes.")

Fishes that have a distinct anterior head with well-developed sense organs and a brain, paired jaws, and fleshy fins that lack internal support rays. Males have a pair of copulatory accessory organs known as claspers. There is no swim bladder, and buoyancy is maintained by retaining high concentrations of urea (a byproduct of protein metabolism) in the blood and muscles. There is a highly electrosensitive system embedded in the skin, each pore being an ampulla of Lorenzinni.

There are two subclasses, the Holocephalii or ratfishes and chimaeras, which have a single pair of gill slits, and the much more diverse Elasmobranchii, containing the sharks, skates, rays, sawfishes and allies. 

Links:

American Elasmobranch Society:  www.elasmo.org/

Australian Museum, Ichthyology Site: austmus.gov.au/fish/

Pelagic Shark Research Foundation: www.pelagic.org 

Sharks, which according to contemporary ichthyologists, includes all the related shark-like fishes such as rays, skates, sawfishes, and guitarfishes. Elasmobranchs are diagnosed by having 5 to 7 pairs of gill slits. Most species also possess a pair of oval or round spiracles above or behind the gill slits. The skin is covered in denticles of similar structure to shark teeth.

This model of a shark skeleton shows how different the cartilaginous structure is from the more familiar bony skeleton of other vertebrates. Notice the solid gill arches and the lack of ribs or rib-like structures.

Superorders:

Galea

Squalea