Osteichthyes

These ancestral forms also gave rise to a group of vertebrates that contain 96% of the living bony fish and the rest of the tetrapods. The bony fish form a large group called Osteichthyes. The common characteristic of bony fish and tetrapods is the presence of endochondral bone (bone tissue that replaces cartilage during development), the presence of lungs or swim bladder that develops from the gut and several dental and cranial characteristics. They are the most successful aquatic vertebrates occupying all possible niches in freshwater, marine and brackish water.

1.0Classification 

• Kingdom: Animalia
• Phylum: Chordata
• Subphylum: Vertebrata
• Division: Gnathostomata
• Superclass: Pisces
• Class: Osteichthyes
• Sub-classes: Two 
• Sarcopterygii- lobe-finned fish
• Actinopterygii- ray-finned fish

2.0Osteichthyes Characteristics

• Another adaptation found in bony fish is the specialization of jaw muscles and skeletal elements for feeding. The mouth is terminal or subterminal, and the jaws usually have teeth.
• Operculum: They have an operculum on either side composed of bony plates attached to muscles covering the gills. This feature increases respiratory efficiency by rotating outwards, thus creating a negative pressure that assists the water to flow along with the mouth pump over the gills. 
• It includes both marine and freshwater fishes with body endoskeleton.
• Their body is streamlined.
• Their exoskeleton is made up of scales, which may be cycloid or ctenoid type.
• i.e., the skin is covered with cycloid/ctenoid scales.
• Respiration is by 4 - pairs of gills. These gills are covered by an operculum on each side.
• The mouth is terminal primarily, and teeth are found in Jaws.
•  Supplementary respiratory structures, "air bladders", are present. A gas-filled diverticulum of the esophagus provides further means of gas exchange in oxygen-deprived water, which is otherwise used for buoyancy. 
• Fish that use this gas-filled diverticulum exclusively for breathing are called lungs, and fish that use this gas-filled chamber for breathing are called the swim bladder. In other fishes, these air bladders help maintain a balance of the body and provide buoyancy.
• The scroll valve in the intestine is absent. ( To increase the Surface area for greater absorption.
• Cloaca absent, in place of a cloacal aperture anus is present.
• No Ampulla of Lorenzini.-Electro receptors
• The tail is of homocercal type.- Both Tail lobes are equal.
• Genital ducts open outside the body through separate apertures.
• Fertilization is usually external, and claspers (copulatory organs) are absent in male fish.
• They are mostly oviparous.- Egg laying. 
• Development is direct.-  No larva stage in the life cycle. 

Examples :

• Hippocampus "Sea horse": It swims in sea water in a vertical position. A pouch-like structure, known as a "brood pouch," is present on the abdomen of male fishes. In this pouch, the male collects the eggs. Secondary vivipary and parental care are found in the Hippocampus.
• Exocoetus  Flying fish.
• Labeo:- "Rohu" or "Indian carp" - It is a freshwater fish.
• Clarias:- "Catfish" or Magur (Freshwater)
• Catla Catla (Fresh water)
• Betta Fighting Fish (Aquarium fish)
• Pterophyllum AngelFish (Aquarium fish)
• Gambusia - Larvivorous fish.
• Latimeria chalumnae (Coelacanth) - Living fossil

3.0Class Actinopterygii

• Actinopterygii, known as ray-finned fish or actinopterygians, is a class of bony fish comprising over 50% of living vertebrate species.
• General Features of Action pterygia 
• Actinopterygians or ray-finned fish have a skeleton with bone of endochondral origin. 
• Caudal fin is heterocercal in ancestral forms but homocercal in descendent forms
• Skin with mucous glands; ganoid scales in ancestral forms but cycloid or ctenoid or absent in derived forms. Scales grow throughout life. 
• Paired fins are thin and broad without fleshy basal lobes but supported by long dermal rays. Their movement is controlled by muscles in the body. 
• Double external nares are present; olfactory sacs do not open in the mouth. 
• Teeth with a covering of enamel; spiral valve present in the stomach in ancestral forms but absent in derived forms. 
• Respiration by gills supported by gill arches and covered by a bony operculum. The swim bladder may or may not be connected to the oesophagus by a duct and usually functions in the fish's buoyancy. 
• The heart consists of sinus venosus, undivided atrium and ventricle, single circulation, aortic arches, and erythrocytes nucleated. 
• The excretory system consists of paired opisthonephric kidneys.  
• The sexes separate. Fertilization is usually external,. The larvae are usually oviparous, rarely ovoviviparous or viviparous. Their larval forms are very different from those of the adults.
• In the nervous system, the brain has a small cerebrum, optic lobes, cerebellum, ten pairs of cranial nerves, and three semicircular canals.

4.0Class Sarcopterygii 

• These are the lobe-finned fish. Sarcopterygii referred to only fish, but it has become clear that the tetrapod ancestor was a sarcopterygian fish. 
• Therefore, the group has been enlarged to include the tetrapods as well.

General Characters of living Sarcopterygian fish

• The caudal fin is diphycercal; paired pectoral and pelvic fins are present, supported by stout bones and bony rays. 
• Muscles control the movement of the fins. 
• Gills covered by bony operculum; swim bladder is present and used for respiration in lungfish. 
• Heart with sinus venosus, atrium, partially divided ventricle and a conus arteriosus. 
• Pulmonary and systemic circuits are incompletely separated. 
• Skeleton ossified: skin covered by bony dermal scales (cosmoid) covered by cosmine, a form of dentine. 
• Jaws with polyphyodont teeth covered by true enamel. Intestine with spiral valve. 
• The brain is small but well-developed, with 10 pairs of cranial nerves and three pairs of semicircular canals. Sexes separate; fertilization is internal in coelacanths and external in lungfish.

Examples: -

• Coelacanth (African lobe fins)
• Protopterus: (African lungfish)
• Lepidosiren: (American lungfish)
• Neoceratodus: (Australian lungfish)

5.0Difference Between Chondrichthyes and Osteichthyes