Question

Placoid scales are formed in:
A. Cartilaginous fishes
B. Bony fishes
C. Paleontical fishes
D. Lung fishes

Answer 1

Placoid scales are tiny and tough scales. They are hard and dentated and covered with enamel. These scales provide streamlined nature.

Complete answer:
Placoid scales are seen in cartilaginous fishes. Cartilaginous fishes are sharks, manta rays, and chimeras.
These scales consist of toothlike scales. These toothlike scales are called as dermal denticles or placoid dentations.
These denticles provide the following two functions:
Protection against predators
Providing a streamlined body that facilitates easy and non-friction motion in the water
These scales are structurally homologous with the teeth seen in invertebrates. They have a central pulp cavity that is supplied with blood vessels. This pulp cavity is surrounded by a conical layer of dentine.

Hence the correct answer is : Option A: Cartilaginous fishes

Additional Information:
Reptilian skin is covered with scutes or scales. They are one of the many other characteristics that distinguish reptiles from the animals of other classes. These scales are made of alpha and beta-keratin. They are formed from the epidermis.

Note: Bony fish are also known as Osteichthyes. They are a diverse taxonomic group of fish that have skeletons made of calcareous bones that are primarily composed of bone tissue. This is opposed to cartilage as seen in cartilaginous fish.
Lungfish are well known for retaining the primitive characteristics within the Osteichthyes. These include the ability to breathe air instead of dissolved oxygen in the water. They also retained certain structures and characteristics that are primitive within the Sarcopterygii. These include the presence of lobed fins along with a well-developed internal skeletal system.
Cycloid or ctenoid scales are seen in bony fish.
Ganoid scales are present in fish like pufferfish, bichirs, bowfin, paddlefishes, gars, sturgeons, etc.
Cosmoid scales in lungfishes and some fossil fish.