Write a short note on class Cyclostomata.

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