Write the characteristics of class $Chondrichthyes$.

(N/A) The class $Chondrichthyes$ (cartilaginous fishes) exhibits the following characteristics:
$1$. They are marine animals with a streamlined body and have a cartilaginous endoskeleton.
$2$. The mouth is located ventrally.
$3$. Notochord is persistent throughout life.
$4$. Gill slits are separate and without an operculum (gill cover).
$5$. The skin is tough,containing minute placoid scales.
$6$. Teeth are modified placoid scales which are backwardly directed.
$7$. Their jaws are very powerful.
$8$. Due to the absence of an air bladder,they have to swim constantly to avoid sinking.
$9$. The heart is two-chambered (one auricle and one ventricle).
$10$. Some of them have electric organs (e.g.,$Torpedo$) and some possess poison stings (e.g.,$Trygon$).
$11$. They are cold-blooded (poikilothermous) animals.
$12$. Sexes are separate. In males,pelvic fins bear claspers.
$13$. Fertilization is internal and many of them are viviparous.