In the life history of Petromyzon a larval form is seen. It is called Ammocoetes larva.
1) Ammocoetes larva is transparent and 1 cm., in length.
2) Ammocoetes larva lives in U shaped burrows of mud.
3) Ammocoetes larva comes out of the burrow at night only to change its feeding ground, then it buries again.
4) Ammocoetes larva has an eel like body and it differs from the adult In several characters.
5) It has no buccal funnel.
6) Around the mouth a semi circular oral hood is present
7) Below the mouth short transverse lower lip is present and it has no teeth.
8) Paired eyes are covered by thick skin.
9) Behind the buccal tentacles, a velum with a pair of cup-shaped muscular flaps are seen.
10) Surrounding the mouth buccal tentacles are present.
11) The pharynx is associated with seven pairs of gill pouches.
12) The pharynx is continued into the oesophagus posterioily.
13) The endostyle is present on the ventral side of the pharynx.
14) The mucus secreted by the endostyle passes into the peripharyngeal grooves of the pharynx.
15) Food particles like unicellular algage, bacteria which enter with the incurrent of water and entangled with the mucous. Thus the food associated with mucous carried into the oesophagus from the pharynx. Because of muscular movement of velum and pharynx, the ammocoetes larvae feeds.
METAMORPHOSIS OF AMMOCOETES LARVA
1) After a long period of larval life, the Ammocoetes larva metamorphoses into the adult.’
2) The endostyle modifies into the thyroid gland.
3) The oral hood changes into the buccal funnel with horny teeth,tongue and a round mouth.
4) Paired eyes are developed completely.
5) The velum degenerates by leaving a rudiment.
6) The continuous dorsal fin breaks into dorsal and a single caudal fin.
7) The young lamprey migrates to the open sea from rives and becomes carnivorous adult animals.
The ammocoetes larva exhibits striking similarities with Amphioxus. it is a very primitive and generalised vertebrate. It may be regarded as the connection link between Amphioxus and cyclostomes.