Subphylum: Vertebrata A jawless group, the Agnathans, and a group that developed jaws, the Gnathostomatans. The modern representatives of the Agnathans are the hagfish and the lampreys.

The cartilaginous fishes, Chondrichthyes, include the sharks, skates and rays. The bony fishes, or Osteichthyes, include two groups, the ray finned and the fleshy finned fishes. The ray-finned fishes represent the majority of the present fish. The flesh finned fishes, the lung fish and the lobe finned fish, are a small almost extinct group that is believed to have provided the ancestral stock for the remainder of the vertebrates.

The Agnathans: represent a small group of organisms; the modern lamprey and hagfish long, slender eel like; median fins but lack swimming control produced by lateral fins. swim in a lateral undulating motion exist either as bottom scavengers or ectoparasites of other fish.

lack of jaws ingest material by a sucking action lost the bony armour utilizes a cartilaginous skeleton.

Gnathostomatan. These fish represent the other major line of vertebrates. difference between the gnathostomatans and the agnathans is the presence of jaws Jaws are thought to have evolved from the modification of the primitive gill arches group includes the Placoderms (extinct), Chondrichthyes, Actinopterygii and the Sarcopterygii.

Chondrichthyes: The Chondrichthyes are the modern cartilaginous fish. arose from an early Gnathostomatan modern members are a small group of marine carnivores but at one time were the dominant aquatic organisms present Chondrichthyes include the sharks, skates and rays have not undergone the same extensive radiation as the bony fish

well developed sense organs, powerful muscles and predacious nature ensure their continued survival Features of this group include: ventral jaw with replaceable teeth limited mobility of the jaw means that the shark must thrash around to break up its prey.

heterocercal tail drives the animal upwards as it swims. This is beneficial because the fish is heavier than water and has no swim bladder for buoyancy (although the large lipid filled liver does provide some resistance to sinking). This means the sharks must continually swim to avoid sinking.

lateral fins are well developed to control pitch but are not for braking. skates and rays have lateral pectoral fins have been greatly enlarged and the animals beat them like wings when swimming. gills are not protected by an operculum. extant Chondrichthyes have a well developed sensory system that permits rapid and accurate detection of danger or prey at a considerable distance.

Actinopterygii: Bony Fishes include all the modern bony, ray-finned fishes. well adapted and successful, occupied almost every conceivable aquatic niche. equally well suited to marine and freshwater environments. The ray-finned fishes represent the largest single group of chordates, with over 20,000 extant species. they live as carnivores, herbivores, parasites and scavengers. -

Some of their more successful features are: The presence of well developed lateral fins with a bony peripheral skeleton to provide delicate articulations. The support for the fins comes from a series of fine delicate bony rays that are attached to and extend from the skeleton.

The homocercal tail (caudal fin) provides a powerful lateral of forward thrust without driving the animal up or down in the water. The pectoral and pelvic fins act as stabilizers and provide a system to generate pitch, yaw, roll and braking as required for highly manoeuvrable and controlled swimming. scales of the Actinopterygians are of light bone and they are small or absent in fast swimmers (reduces drag).

Drag during swimming is also reduced by the streamlined (teardrop) shape and the production of surface mucous. the swim bladder, is a major development to the bony fishes. It provides the fish with a neutral buoyancy. This in turn, permits the fishes to hover in the water and not have to swim constantly and thus expend energy to avoid sinking. The reduction in energy expenditure reduces the amount of time required for feeding.

Respiration in the aquatic environment is difficult for two reasons. The medium contains far less oxygen per unit volume than air, and its viscosity makes it more difficult to move during irrigation of the respiratory surfaces or gills. The bony fishes have developed extensive gills to maximize the surface area for gas exchange. They use a counter current system where the blood in the gill capillaries flows in the opposite direction to the water flowing over the gills. This allows for a higher saturation of the blood with oxygen. bony operculum provides a degree of protection for the delicate gill tissues and by moving the operculum, water can be forced over the gills without the fish being compelled to move.

Other systems within the bony fishes are not as well advanced. The circulatory system has only a two chambered heart and the blood flow to the body is under very low pressure. The brain is poorly developed relative to higher chordates. The sensory and to some extent the motor coordinating centers of the brain are well developed, but the overall coordinating and reasoning centers are primitive.

Sarcopterygians: a small ancient group of fishes of which the majority are extinct. These are the `fleshy-finned' fishes and the two major types are the lobe-finned fish and the lungfish. There are only four living genera of the Sarcopterygians. The significance of the group comes from their suspected contribution to tetrapod evolution. The lobe- finned fish are thought to be the forerunners of the terrestrial animals.

Coelacanth

The first living lungfish was discovered in the 1830s, almost a century before the coelacanth appeared. Like the coelacanth, this fish was previously thought to be extinct, because it was known only from fossilized Devonian-Era remains almost 450 million years old.

There is only one living species of bowfin, which is also called the dogfish, mudfish, or grindle. …discovered fossilized bowfin from North America, South America, Europe, Africa, and Asia, the earliest of which dates to the Jurassic Era, which began 213 million years ago.