cartilaginous fish

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Fish in which the skeleton is made of cartilage. Sharks, rays, and skates are cartilaginous. Their scales are placoid (isolated structures made of dentine resembling simple teeth) and are present all over the body surface. The scales do not continue to grow once fully formed, but are replaced by new scales as they wear out. The notochord (primitive skeletal rod) is reduced and replaced to varying degrees by cartilage.

In modern cartilaginous fish, lungs or swimbladder-like structures (which help to keep fish at a certain depth) are absent, and the fish must maintain its level in the sea by the muscular efforts of swimming. Claspers (fins modified for mating) are sometimes present in the male.

Classification
There are about 900 species in the class Chondrichthyes, divided into two subclasses. Subclass Holocephali has one order of living fish, Chimaeriformes, consisting of three families of the chimaera. These have cartilaginous skeletons like sharks, but have only one gill opening with a single gill-cover, like the bony fishes.

Subclass Elasmobranchii contains the eight orders of sharks, and the superorder Batoidea, comprising the skates and rays.

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