Marine animals with streamlined body<|>Cartilaginous endoskeleton<|>Notochord is persistent throughout life<|>Mouth is located ventrally, with powerful jaws<|>Gill slits are separate and without operculum<|>Skin is tough, containing minute placoid scales<|>Teeth are modified placoid scales which are backwardly directed<|>Due to the absence of air bladder, they have to swim constantly to avoid sinking<|>Heart is two-chambered (an auricle and a ventricle)<|>They are cold-blooded (poikilothermous) animals (They do not regulate body temperature)<|>Sexes are separate<|>In males, pelvic fins bear claspers<|>Internal fertilization<|>Many of them are viviparous
