Class Chondrichthyes

Chondrichthyes (Greek chondros, cartilage; ichthys, fish) are marine fishes whose entire endoskeleton is built from cartilage rather than bone, with the notochord persisting throughout life. The skin is tough, covered by dermal placoid scales, and the caudal (tail) fin is heterocercal — asymmetrical both in its external shape and its internal skeletal support.

The mouth sits on the ventral (underside) surface, and the teeth are simply modified, backward-pointing placoid scales — a direct structural link between skin covering and dentition that is unique to this class. Their jaws are powerful, and most species are predaceous. Respiration takes place through lamelliform gills that lack a protective operculum (gill cover), unlike the bony fishes. Excretory organs are mesonephric kidneys, and the heart is two-chambered. …