GENERAL ANATOMY

Sharks are fish that have no bones, only cartilage. They have 5-7 gills (without gill covers, operculum) in front of their pectoral fins (on both sides). Shark tails are asymmetrical; the top lobe of tail is larger than the bottom lobe. The shark's jaw is not fused to the braincase and can enlarge to eat very large prey. Sharks have no swim bladder for buoyancy (like the bony fishes); an oily liver aids buoyancy. Sharks have an advanced electroreceptive system that detects slight electrical fields.

The teeth and skin are modified placoid scales. A spiracle (a special gill slit that supplies oxygen directly to the eyes and brain) is present in front of the first gill. Sharks' teeth are replaceable when they've broken or worn out. Their intestines are short and compact and have an unusual structure; a spiral valve (shaped like a spiral staircase) fills the cylindrical intestines and allows absorption of the food in a short span of intestine.

SOURCE: http://www.enchantedlearning.com/subjects/sharks/