There are two classes of fishlike creatures in the Chordate Phylum. The first is Chondrichthyes Class, which comprises mainly of Sharks and Rays. The seconds is the common Bony fishes of the class Ostechithyes. The apparent similarities between each speech are only skin deep. They have similar dapperly solely because of convergent evolution and not because of any close evolution connection.
There paired fins can identify class Chrondrichthyes, which comprises namely of sharks and rays, their completely cartilaginous skeletons and biting jaws characteristic of the group. Chrondrichthyes are mainly carnivorous, and the sharks have highly adept senses for vision, olfaction, and hearing. They even posses a system of microscopic organs which can detect changes in water pressure around the shall. Sharks do not possess gills flaps like a fish so a shark must continuously move in order to obtain enough oxygen to survey. The large teeth of a shark are evolutionarily derived from jagged skin scales. Which are apparent on shark s ancestor s Placodermi class. The digestion system of a shark contains a spiral valve intestinal system, which increases the surface area and lengthens the time food is digested in the unusually short intestine system of a shark. Sharks sexually reproduce. Unlike a shark, rays have flattened bodies so they can hide themselves in the sand at the bottom of any shallow water area and wait for a meal. Rays also have a whiplike tail for defense and jaws, which it uses to crush mollusks and crustaceans.
Class Osterrichhthyes has the most separate species of any vertebrate class. Unlike their oceanic cosines Bony fish have a skeleton made from calcium phosphate and a slimy skin covered by bony armorlike scales. Fish of the Osterrichhthyes class have gill flaps call an operculum. On the side of there head which have flaps that move while the fish is stationary drawing in water which the fish can convert into oxygen. An unusual characteristic of bony fish is that they can control their density and this control their buoyancy by means of a unique swim bladder. Fish system of flexible fins in another adaptation the fish have made. Unlike sharks bony fishes fertile externally and even in the Osterrichhthyes Class there is great variation in reproductive techniques. There are two distinct subclasses of fishes, Ray finned and fleshy finned, each has a common ancestor. Though divergent evolution the ray finned fish have become a separate class from the fleshy finned. The more common of the two is the ray-finned fish. Example of this is the trout, perch, tuna, and herring. The finned of this subclass are supported mainly by long flexible rays, which have become modified for maneuvering, defense, and other functions over time. Fleshy fish have nuscykar ecitrak and pelvic fins, which are supported by an extension of there bony skeleton. Many fleshy finned fishes are bottom dwellers which uses there paired finned to : walk on the substrate under water. Fleshy-finned fish have a strong connecting to the Class Amphibian, which can be seen by the homologous skeletal structures.