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

Chapter 4. DIVISION GNATHOSTOMATA, or ECTOBRANCHIATA, GNATHOSTOMATA, seu ECTОBRANCHIATA. SUPERCLASS Pisces

4.2. CLASS BONY FISHES OSTEICHTHYES

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

4.2.3.2. Subclass Ray-finned Fishes Actinopterygii

This subclass comprises the majority of modern Fishes—approximately 20,000 species; representatives of this subclass inhabit all types of Water bodies, from multi-kilometer ocean depths and salt lakes to streams and underground springs.

The scales are ganoid, or bony—cycloid or ctenoid. In some species, the scales fuse to form bony plates; in others, 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 from 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 the 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, the ray-finned fishes. The conus arteriosus in The Heart is present only in representatives of ancient groups; in teleosts, 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. The spiral valve of the intestine disappears, persisting in a reduced form only in ancient groups, while the intestine itself elongates. The 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 ray-finned fishes have been discovered in freshwater deposits of the Middle Devonian in various regions of Europe and Asia; since the Carboniferous period, they have inhabited all water bodies of the globe. Evolution, lasting 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 contains only extinct forms), comprising 35–60 orders (10–16 orders consisting only of extinct forms).