Question

Placoid scales are found in
A) Reptiles
B) Bony fishes
C) Cartilaginous fishes
D) Amphibians

Answer 1

Hint: The scale extends forward and inward and it is the cusps which are liable for the shark's skin's roughness.

Complete Answer:
A type of scale containing the basic element of the hard skin shield of sharks is the placoid scale. It comprises a strong skin-embedded base and a spiny layer; vitrodentine protects these systems.

Now, let us find the solution from the options-
- There are epidermal scales for reptiles. Horny, harsh extensions of the stratum corneum are epidermal scales. Well established in reptiles, they are popular in mammals and birds on exposed skin as well. These scales, together with the remainder of the stratum corneum, are regularly moulted or shed gradually. Thus option A is not the correct option.
- Fish have dermal scales on them. In the transverse ridges of the skin, dermal scales are tiny flat discs set in pockets. Many other scales of varying sizes are included in each pocket. Thus, option B is not the correct option.
- Characteristically, cartilage fishes possess placoid scales. The skin of a cartilaginous fish is protected by placoid scales and dermal denticles, tooth-like scales distinct from those seen on bony fish that are flat scales.
- Typically, amphibians do not possess scales. Thus, option D is not the correct option.

Thus, the correct answer is option (C) Cartilaginous fishes.

Note: The bulk of the scale comprises a core pulp cavity surrounded by dentin. Placoid scales avoid growing when they exceed a certain size, unlike the bony fish scales, and new scales are introduced as the animal grows.