Question

what are the modification that takes place in ammocoete larva of petromyzon

Answer 1

Petromyzon, also called as Lamprey are the jawless fishes of the order Petromyzontiformes. The adult fishes have a toothed, funnel-like sucking mouth.

The young larvae called ammocoetes, spend several years in the rivers, where they live burrowed in fine sediment, filter feeding on detritus and microorganisms. Then, the ammocoetes undergo a metamorphosis that lasts for several months.

The metamorphosis brings changes in the head region. The endostyle gets modified into the thyroid gland. The oral hood changes into the buccal funnel with horny teeth, tongue and round mouth. A pair of eyes is developed completely. The velum degenerates by leaving a rudiment. The continuous dorsal fin breaks into dorsal and a single caudal fin.

The young lamprey migrates to the open sea from rivers and they become carnivorous adult animals.

Answer 2

The larval stage of an ammocoetes larva or primitive lamprey illustrates the concept of ontogeny recapitulating phylogeny which means that its development expresses all the intermediate forms of its ancestors throughout evolution.

It displays many characteristics of ancient chordates as well as the embryological traits of the vertebrates. It has gill slits, myomeres or body muscle segments, a notochord, a dorsal nerve cord, a dorsal fin, a primitive nervous system with a brain and eyes.