Write the characteristics of class $Cyclostomata$.

(N/A) The characteristics of class $Cyclostomata$ are as follows:
$1$. All living members of the class $Cyclostomata$ are ectoparasites on some fishes.
$2$. They have an elongated body bearing $6-15$ pairs of gill slits for respiration.
$3$. $Cyclostomes$ have a sucking and circular mouth without jaws.
$4$. Their body is devoid of scales and paired fins.
$5$. The cranium and vertebral column are cartilaginous.
$6$. Circulation is of the closed type.
$7$. $Cyclostomes$ are marine but migrate for spawning to fresh water. After spawning,within a few days,they die. Their larvae,after metamorphosis,return to the ocean.