Question

Which type of scales are found on the skin of cartilaginous fishes?
(a)Cycloid
(b)Ctenoid
(c)Gonoid
(d)Placoid

Answer 1

Hint: A small rigid plate that grows out of a fish's skin is a fish scale. These protective scales, which can also provide effective camouflage through the use of reflection and coloration, as well as possible hydrodynamic benefits, cover the skin of most fishes.

Complete answer:
-The size, shape, structure, and extent of the scales vary enormously, ranging from strong and rigid armour plates in fish, such as shrimpfish, to microscopic or absent in fish, such as eels and anglerfish.
-The cycloid scales of salmon and carp, or ctenoid scales of perch, or the ganoid scales of sturgeons, cover most bony fishes.
-Cartilaginous fishes (sharks and rays) are coated with placoid scales.
-Often called dermal denticles, the placoid scales of cartilaginous fishes are structurally homologous to vertebrate teeth.

Additional Information:
-Instead, some animals are covered by scutes, while others on part or all of the skin have no outer coating.
-The scales of bony fish have been proposed to be similar in form to teeth, but they are likely to come from different origins.
-Most fish are often coated in a film of mucus or slime that can defend against pathogens such as bacteria, fungi, and viruses and minimize surface resistance when fish swim.
-Fish scales are part of the integumentary system of fish and are developed from the dermis mesoderm layer, which distinguishes them from reptile scales.
-The same genes that are involved in mammalian tooth and hair growth are also involved in the development of scales.

So, the correct answer is ‘Placoid’.

Note:
-It is possible to use the morphology of a scale to identify the fish species.
-The word scale is derived from "escale" from the Old French, meaning a shell pod or husk.