There are two classes of fish-like creatures in the Phylum Chordates. The first is the class Chondrichthyan, which mainly includes sharks and rays. The second is the common bony fish of the class Ostechithyes. The apparent similarities between each speech are only superficial. They have a similar appearance only because of convergent evolution and not because of a close evolutionary connection. There, paired fins can identify the class Chrondrichthyes, which includes sharks and rays among others, their completely cartilaginous skeletons and biting jaws characteristic of the group. Chrondrichthyes are primarily carnivorous and sharks have highly developed senses for vision, olfaction and hearing. They even have a system of microscopic organs capable of detecting changes in water pressure around the site. Sharks don't have gill flaps like fish, so they must continually move around in order to get enough oxygen to be studied. A shark's large teeth derive evolutionarily from jagged skin scales. Which are apparent on the Placodermi class of shark ancestor. A shark's digestive system contains a "spiral valve"; intestinal system, which increases the surface area and prolongs the digestion time of food in a shark's unusually short intestinal system. Sharks reproduce sexually. Unlike sharks, rays have a flattened body that allows them to hide in the sand at the bottom of any shallow water area and wait for a meal. Stingrays also have a whip-like tail for defense and jaws, which they use to crush molluscs and crustaceans. The class Osterrichhthyes has the most distinct species of all vertebrate classes. Unlike their oceanic cosines, bony fish have a skeleton made of calcium phosphate and slimy skin covered in bony, armor-like scales. Fish of the class Osterrichhthyes have gill flaps called operculum. On the side of the head, there are flaps that move while the fish is stationary, drawing in water that the fish can convert into oxygen. An unusual feature of bony fish is that they can control their density and thus their buoyancy by means of a unique swim bladder. Fish system consisting of flexible fins in another adaptation that fish have made. Unlike sharks, bony fishes are externally fertile and even within the class Osterrichhthyes there is wide variation in reproductive techniques. There are two distinct subclasses of fish, the ray-finned fish and the fleshy-finned fish, each of which has a common ancestor. Although their evolution is divergent, ray-finned fishes have become a distinct class from fleshy-finned fishes..