Describe reproduction in Cyclostomata.

(N/A) $\rightarrow$ Cyclostomes are marine but migrate to fresh water for spawning.
$\rightarrow$ After spawning,they die within a few days.
$\rightarrow$ Their larvae,after metamorphosis,return to the ocean.
$\rightarrow$ Examples: $Petromyzon$ (Lamprey) and $Myxine$ (Hagfish).