Do any one knows how many sets of gils the shark have?
and what did become of them during the evoluion .
Guy

Yes,
Sharks has got 3.5 set of gills on each side of their head.
Later on during the evolution, 3 of them become the auditory bones.
Maleus, Incus, and Stapes if I can recall corectly the names

As far as I know, sharks have 5 gill slits, and being primitive animals that have survived through evolution, I feel the number hasn't increased or decreased. The respiratory surface was and is the walls of the gill slits which are highly vasculated. Some say that during the course of evolution, sharks migrated from fresh water to sea water but I feel they lived in sea water all their lives. The only evolution that they might have undergone is a reduction in size. The fact that they have glomerular kidneys instead of aglomerular kidneys might indicate the condition of the sea water at the time of their origin. The water at that time may have been more dilute and so removal of excess water may have required them to have glomerular kidneys, and now that the conditions have changed, their kidneys remain the same but they retain urea in their bodies instead, to prevent excess loss of water from the body.

Thanks for the information and the correction, ideed shark have between 5 to 7 gill arches.
G

Unlike bony fish, elasmobranchs like the shark have gill slits that are actually openings of the pharynx that extend outside... Inside the walls of the openings is present the blood vessels that carry oxygenated and deoxygenated blood... There are altogether 5 to7 pairs of gil slits but the first one is always a spiracle or a pseudobranch which is supplied with oxygenated blood instead of deoxygenated blood... This is because this opening is used as an inlet for water when the animal is at rest and the mouth is closed, in order to continue the respiratory process