Chondrichthyes

The class Chondrichthyes is also known as Elasmobranchia. It includes the sharks, skates, and rays. They are termed "cartilliganeous fish."

Sharks

Sharks do not have swim bladders, and if they were to stop moving they would probably sink to the bottom of the ocean; therefore, they are usually constantly in motion. Sharks usually are scavengers, eating injured fishes, carrion, garbage, and other waste from ships as well as animals such as seals, turtles, birds, whales, crabs, and a wide range of fishes. The whale shark is the largest shark and also the largest fish in the sea, measuring up to 15 m (49 ft) in length; the cookie-cutter shark measures less than 50 cm (19 in) in length. Usually there are five gill slits that lie behind the head. The tail's symmetry is termed cercal because of its irregular shape. (1)

Pictures

A Hammerhead Shark
A (*) Whale Shark
Another Great White Shark.
JAWS!--attacking us!
A Sand Shark.

Skates and Rays

Skates are a family of flat-bodied rays found in warm and temperate seas. They are in the family Rajidae of the order Rajiformes.
Rays have broad, flattened bodies, with eyes located on the upper surface of the body while the mouth and gills are located on the lower surface. The upper surface of the body is dark colored, while the lower surface is light colored. This is called countershading and is a technique many marine animals use for protection as well as for predation. Countershading makes the animal almost invisible because when another animal looks down at them, they are camouflaged with the darkness of the sea bottom. When looked up at from underneath, the animal is camouflaged with the light from the sun. Great White Sharks use this technique for predation. Skates and rays swim using rajiform swimming.

* A picture of two skates taken by me at the New England Aquarium

(1) "Shark," Microsoft Encarta 96 Encyclopedia. 1993-1995 Microsoft Corporation

Information provided by: http://www.cyhaus.com