Chordata: Class Chondrichthyes

Class Chondrichthyes

The almost exclusively marine sharks, rays, and chimaeras of the class Chondrichthyes have skeletons made of cartilage. The mouth, equipped in most sharks with numerous sharp teeth, is located on the underside of the head. Passages called gill arches lead from the pharynx to the exterior and are lined with gill filaments. The gill arches are supported by gill bars. Except in chimaeras, the external gill slits are not covered and are conspicuous on the surface of the body. The jaw consists of two distinct pieces; the upper part is not fused to the braincase as in higher vertebrates. The tail is asymmetrical, curving upward in a shape found in early fossil fishes and thought to be primitive. There is no lung or swim bladder. The skin is studded with toothlike structures called denticles. Sharks have typical vertebrate kidneys that excrete a very dilute urine consisting mostly of water; presumably the earliest vertebrates (ancestral to sharks) evolved in freshwater, where this function is necessary to maintain the correct concentration of the physiologically important salts in the tissues against the tendency for them to be diluted by the inward diffusion of water. In marine species, on the other hand, it is necessary to prevent the concentration of those salts from increasing. Although the kidneys of sharks pump out water, their body fluids contain ammonia in concentrations high enough to make the osmotic pressure equal to that of seawater; this prevents the inward diffusion of salts. Sharks have internal fertilization and lay large eggs, well supplied with yolk and protected by leathery shells. In a few species the eggs are hatched within the body.

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