Describe class - Chondrichthyes.

(N/A) $\rightarrow$ Chondrichthyes are marine animals with a streamlined body and a cartilaginous endoskeleton.
$\rightarrow$ The mouth is located ventrally.
$\rightarrow$ The notochord is persistent throughout life.
$\rightarrow$ Gill slits are separate and without an operculum (gill cover).
$\rightarrow$ The skin is tough, containing minute placoid scales.
$\rightarrow$ Teeth are modified placoid scales which are backwardly directed.
$\rightarrow$ Their jaws are very powerful.
$\rightarrow$ These animals are predaceous.
$\rightarrow$ Due to the absence of an air bladder, they have to swim constantly to avoid sinking.
$\rightarrow$ The heart is two-chambered (one auricle and one ventricle).
$\rightarrow$ Some of them have electric organs (e.g., Torpedo) and some possess a poison sting (e.g., Trygon).
$\rightarrow$ They are cold-blooded (poikilothermous) animals, i.e., they lack the capacity to regulate their body temperature.
$\rightarrow$ Sexes are separate.
$\rightarrow$ In males, pelvic fins bear claspers.
$\rightarrow$ Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray).