Question: From NCERT 100% NCERT covered from Particular Keyword. NEET 2015 Explanations are 100% from NCERT | [Difficult level: Easy]
A jawless fish, which lays eggs in fresh water and whose ammocoetes larvae after metamorphosis return to the ocean is
(1) Eptatretus
(2) Myxine
(3) Neomyxine
(4) Petromyzon
Answer:(4) Petromyzon.
👉Class – Cyclostomata
All living members of the class Cyclostomata are
ectoparasites on some fishes. They have an
elongated body bearing 6-15 pairs of gill slits
for respiration. Cyclostomes have a sucking and
circular mouth without jaws (Fig. 4.18). 
Their
Their
body is devoid of scales and paired fins.
Cranium and vertebral column are
cartilaginous. Circulation is of closed type.
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.
Examples: Petromyzon (Lamprey) and Myxine
(Hagfish).
