• Sharks are cartilaginous fish (Class Chondrichthyes), i.e. their skeleton is made of flexible cartilage, unlike bony fish.
• The skin of sharks is covered with placoid scales, with a small pointed spine facing backwards and a rigid base. For this reason, the rough skin of these animals is reminiscent of sandpaper.
• Teeth are embedded in the gum and not attached to the jawbone. They are never toothless because they have several layered rows of teeth: when a tooth in the front wears out or breaks, it is replaced by a new one from the back rows. Their teeth have serrated edges, which they use for tearing, cutting, and shredding.
• While the gills of bony fish are covered by opercula, in sharks they communicate with the outside through gill slits.
• Bony fish have an organ that helps control their buoyancy, called the swim bladder. Sharks do not have this, so they are forced to swim continuously so as not to sink.