Major Characteristics of Class Osteichthyes:

  1. Skeleton more or less bony; vertebrae numberous; tail usually homocercal.
  2. Most have skin with embedded dermal scales, some fish without scales.
  3. Paired fins with fin rays of cartilage or bone.
  4. Jawed mouth terminal; most have many teeth.
  5. Respiration by gills supported by bony gill arches and covered by a common operculum.
  6. Swim bladder often present.
  7. Mouth with many teeth (some toothless);jaws present.
  8. Sexes separate and fertilization is usually external.
  9. Circulation consists of a two chambered heart,arterial and venous systems,and four pairs of aortic arches.
  10. Nervous system of brain with small olfactory lobes and cerebrum;large optic lobes.
  11. Most species are oviparous and fertilization is external.
The streamlined shape of a fish and the mucoid secretions that lubricate its body surface reduce friction between the fish and the water.Water's buoyant properties also contribute to the efficiency of a fish's movement through the water.Fishes move through the water using their fins and body wall to push against the incompressible surrounding the water.Because they contain muscle bundles that extend posteriorly and anteriorly in a zigzag fashion,contraction of each muscle bundle can affect a relatively large portion of the body wall.

Nutrition and digestion
Most modern fishes are predators and spend much of their lives searching for food.Their prey vary tremendously.Some fishes feed on invertebrae animals floating or swimming in the plankton or living in or on the substrate.Many feed on other vertebrates.Similarly, the kinds of food that one fish eats at different times in its life varies.Fishe usually swallow prey whole.Teet capture and hold prey, and some fishes have teeth that are modified for crushing the shells of molluscs or the exoskeletons of arthropods.To capture prey,fishes often use suction that closing the opercula and rapidly opening the mouth creates,which develops a negative pressure that sweeps water and prey inside the mouth.
An enlargement,called the stomach,stores large,often infrequent,meals.The small intestine,however,is the primary site for enzyme secretion and food digestion.

Circulation and gas exchange
In fishes,blood flows from the venous system through the thin-walled sinus venosus into the thin-walled,muscular atrium.From the atrium,blood flows into a larger,more muscular ventricle.The ventricle is the primary pumping structure.Anterior to the ventricle is the conus arteriosus,which connects to the ventral aorta.In teleosts,the conus arteriosus is replaced by an expansion of the ventral aorta called the bulbus arteriosus.Blood carried by the ventral aorta to afferent vessels leading to the gills.These vessels break into capillaries and blood is oxygenated.Blood is then collected by efferent vessels,delivered to the dorsal aorta,and distributed to the body where it enters a second set of capillaries.Blood then returns to the heart through the venous system.
Gas exchange across gill surfaces is very efficient.Gill arches support gills.Gill filaments extend from each gill arch and include vascular folds of epithelium,called pharyngeal lamellae.Branchial arteries carry blood to the gills and into gill filaments.The arteries break into capillary beds in pharyngeal lamellae.Gas exchange occurs as blood and water move in opposite directions on either side of the lamellar epithelium.
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