Vertebrate Zoology - V. M. Konstantinov 2011

Chordates
Gnathostomes
Class Chondrichthyes

Biologically, fish occupy a significantly higher evolutionary level than cyclostomes. This is reflected in their vigorous locomotion, active prey capture, greater behavioral diversity and environmental responsiveness, and various adaptive mechanisms to their habitats. These biological traits are associated with the advanced development of numerous Organ Systems, primarily The Nervous system, Sense Organs, and Skeleton. Unlike cyclostomes, the Mouth of fish is equipped with movable jaws. They possess paired appendages represented by pectoral and pelvic fins along with their girdles. For the majority of species, gills serve as the lifelong respiratory organs, with gill filaments being of ectodermal origin. The olfactory organs are paired. The Inner ear features three semicircular canals. As a rule, the body is covered with scales.

Fish are of immense importance in the global ecology of nature as the primary consumers of aquatic plant biomass and lower-tier animals that thrive on this organic matter. Fish generate commercially valuable products (meat, fat, etc.) utilizing biomass that is not yet directly involved in human economic activity.

Fish originated in freshwater bodies, apparently during the Silurian period (as evidenced by fossil bone fragments), and by the Devonian they constituted the dominant group of animals across all aquatic basins. At the beginning of the Devonian period (or potentially as early as the Silurian), the divergence of fish into several branches began, of which two major classes have survived to the present day: Chondrichthyes (Cartilaginous fish) and Osteichthyes (Bony fish).

General characteristics

Cartilaginous fish—a relatively small modern group comprising about 730 species—combine predominantly primitive organizational traits with progressive ones.

The skeleton remains cartilaginous throughout life.

The Skin is covered with the most primitive type of scale, namely placoid scales (though skin may occasionally be naked). There are numerous gill slits (5 to 7), and each opens independently to the exterior as a slit-like orifice (with chimaeras being the exception).

Cartilaginous fish are characterized by such progressive features as the presence of Nervous Tissue in the roof of the Forebrain, internal Fertilization, and viviparity in many species.

Body sizes vary widely: from 20 cm up to 15 or even 20 meters. They are distributed across all seas (except the Caspian Sea) and oceans, predominantly in tropical latitudes. Some species venture into rivers. They are of economic significance in commercial fisheries.

The Class Chondrichthyes is divided into two subclasses: Elasmobranchii (elasmobranchs) and Holocephali (chimaeras).