Chondrichthyes are marine animals with a streamlined body and a cartilaginous endoskeleton. The mouth is located ventrally. The notochord is persistent throughout life. Gill slits are separate and without an operculum, which is the gill cover. The skin is tough and contains minute placoid scales. Teeth are modified placoid scales that are directed backwards. The jaws are very powerful, and these animals are predaceous. Due to the absence of an air bladder, they have to swim constantly to avoid sinking. The heart is two-chambered, with one auricle and one ventricle. Some of them have electric organs, as in Torpedo, and some possess a poison sting, as in Trygon. They are cold-blooded (poikilothermous), that is, they lack the capacity to regulate their body temperature. Sexes are separate. In males the pelvic fins bear claspers. They have internal fertilisation and many of them are viviparous. Examples include Scoliodon (dog fish), Pristis (saw fish), Carcharodon (great white shark) and Trygon (sting ray). The main contrasts with bony fishes are the cartilaginous skeleton, placoid scales, ventral mouth, absence of operculum and air bladder, and internal fertilisation, all of which are repeatedly tested in the exam.