Question

Sol. Key concept: Placoid and ctenoid scales distinguish the illustrated fishes.

X is a shark, a cartilaginous fish. Its characteristic scales are placoid scales or dermal denticles. These tooth-like structures have a basal plate and a projecting spine, with dentine around a pulp cavity and a hard external covering.

Y is the climbing perch, Anabas, a bony fish. It has ctenoid scales, which possess small comb-like projections or ctenii along their exposed posterior margins.

Cycloid scales have a smoother posterior edge; ganoid scales are hard and often rhomboid. The matching X–placoid and Y–ctenoid is therefore option (b).