There are two classes of fish-like creatures in the chordate phylum. The first is the chondrichthyans class, which mainly includes sharks and rays. The second are the common bony fishes of the class Ostechithyes. The apparent similarities between each speech are only superficial. They look similar only because of convergent evolution and not because of a close evolutionary connection. The paired fins can identify the class Chrondrichthyes, which includes sharks and rays, their completely cartilaginous skeletons and the biting jaws characteristic of the group. Crondrichti are primarily carnivorous, and sharks have very keen senses for sight, smell, and hearing. They also possess a microscopic organ system that can detect changes in water pressure around the will. Sharks do not possess gill flaps like fish, so a shark must continually move to get enough oxygen to observe. A shark's large teeth are evolutionarily derived from jagged skin flakes. Which are evident in the Placoderm class of the shark ancestor. A shark's digestive system contains a "spiral valve"; intestinal system, which increases the surface area and lengthens the time it takes to digest food in a shark's unusually short intestine. Sharks reproduce sexually. Unlike sharks, stingrays have flattened bodies so they can hide in the sand at the bottom of any shallow water area and wait for a meal. Stingrays also have a whip-shaped tail for defense and jaws, which they use to crush mollusks and crustaceans. The class Osterrichhthyes has the most distinct species of any vertebrate class. Unlike their oceanic cosines, bony fishes have a skeleton made of calcium phosphate and slimy skin covered in armor-like bony scales. Fish in the class Osterrichhthyes have gill flaps called opercula. On the sides of the head there are fins that move while the fish is still, sucking in water that the fish can convert into oxygen. An unusual feature of bony fish is that they can control their density and this their buoyancy by means of a unique swim bladder. Flexible fin system for fish in another adaptation made by fish. Unlike sharks, bony fishes are externally fertile, and even within the class Osterrichhthyes there is great variation in reproductive techniques. There are two distinct subclasses of fish, ray-finned and flesh-finned, each having a common ancestor. Despite divergent evolution, ray-finned fishes have become a separate class from fleshy-finned ones.