Question

What is the type of scale present in cartilaginous fishes.

Answer 1

Hint: Chondrichthyes class is composed of the majority of the cartilaginous fishes. They are further divided into two subclasses of elasmobranch and Holocephali. The cartilaginous fishes have paired fins, nares, and the presence of scales on their body. They have their skeleton made up of cartilage rather than the bone.

Complete answer:
The scales present on the fishes can be divided into the different types on the basis of their structure.
- Cosmoid scales: these are found in the extinct Crossopterygii and Dipnoi. The external layer of scale is thin and is called vitrodentine. The middle layer present is made up of the hard and non-cellular dentine material and is called cosmine. It consists of branching tubules and chambers present. The innermost layer is made up of the bony substance, isopedine. These scales grow in the addition of the new isopedine material.
- Ganoid scales: the fishes that have these scales are known as the ganoid fishes. These scales are bulky and have a hard outer layer of ganoine. Ganoine is an inorganic material that is enamel-like. The middle layer is cosmine and is composed of the branching tubules. The innermost layer is very thick and is made up of isopedine. These scales grow from the lower and the upper surface of the scales. They provide complete protection to the fishes.
- Placoid scales: these scales are present on the sharks only. Each scale is a disc-like structure which is attached to the base and the spine projecting outward through the epidermis. They have a hard covering of the enamel-like substance vitrodentine. Then comes the layer of dentine and which has several branching dentine tubules. They lie very close to each other and impart a sandpaper-like roughness.
- Cycloid scales: these scales are thin, flexible, translucent plates, circular in shape, with the presence of concentric lines of growth which are used to determine the age of the fish. The central part of the cycloid scale is called the focus. They form a protective covering of the skin.
- Ctenoid scales: they are circular in shape and have more space between them. Spines are present on the posterior area of the scale. They are firmly attached to the skin of the fishes and are free at the limb part.

Note: Certain fishes may have both cycloid and the ctenoid scales present on them. The scales provide protection to the skin of the fishes. They are also used to study the age of the fishes. the scales can be divided into placoid and non-placoid scales.