Chordate Zoology - textbook - Y. V. Tsaryk - 2013

Chapter 4. DIVISION GNATHOSTOMATA, or ECTOBRANCHIATA, GNATHOSTOMATA, seu ECTОBRANCHIATA. SUPERCLASS FISH, PISCES

4.2. CLASS BONY FISHES, OSTEICHTHYES

4.2.3. Systematics of the Class Bony Fishes and Characteristics of Representatives of Individual Orders

4.2.3.2. Subclass Ray-finned Fishes, Actinopterygii

This subclass includes the majority of extant Fishes—approximately 20 thousand species; representatives of this subclass inhabit all types of aquatic environments, from multi-kilometer ocean depths and saline lakes to streams and subterranean springs.

The scales are ganoid, or bony—cycloid or ctenoid. In some species, fused scales can form bony plates; in some species, the Skin is naked (due to scale reduction). A well-developed notochord in a dense sheath is preserved only in certain representatives of ancient groups; typically, amphicoelous bony vertebrae develop. The squamosal bone is absent in the dermal BONES OF THE Skull. The skull is hyostylic. Choanae are absent. The tail is heterocercal in ancient groups and homocercal in more recent groups. The internal Skeleton of paired fins lacks a central axis and is formed by radially arranged elements. The lobes of paired and unpaired fins are supported by

bony rays—lepidotrichia, hence the name of the subclass—ray-finned fishes. A conus arteriosus in The Heart is present only in representatives of ancient groups; in teleost fishes, only two Valves remain of the conus arteriosus, while a thickening of the walls of the initial part of the aorta—the bulbus arteriosus—develops simultaneously. The spiral valve of the intestine disappears, being preserved in a reduced form only in ancient groups, while the intestine itself elongates. A cloaca is absent; the intestine ends in an anus. The swim bladder develops as an outgrowth of the dorsal part of the Esophagus; in some species, it is secondarily reduced.

Fossil remains of actinopterygians have been discovered in freshwater deposits of the Middle Devonian in various regions of Europe and Asia; by the Carboniferous period, they already inhabited all Water bodies of the globe. Their evolution, spanning at least 320-350 million years, has resulted in a great diversity of ray-finned fishes. The systematics of the subclass is complex and not fully resolved. The subclass is most commonly divided into 10-12 superorders (one of which consists entirely of extinct forms), comprising 35-60 orders (10-16 orders being exclusively extinct forms).