Not every shark is a fear-inspiring giant, yet some are even more than that. They range in size from two feet to about sixty feet in length. We encounter them in ocean waters throughout almost all of the world, but seldom where it is extremely cold. A few species run up into the fresh waters of tropical rivers, and three or four are regular inhabitants of fresh-water lakes in Central America and Thailand.
Sharks are flesh-eaters, and some are extremely savage in their habits, as we shall soon see. Most of them give birth to living young, but a number lay eggs with horny cases; fertilization is always internal. You may be surprised at the enormous variety among the sharks: about 235 species are known.
Most of the skates and rays live in the sea, although the sawfishes and some of the skates go far upstream, and some of the rays live permanently in fresh water. The bodies of these strange animals are very much flattened, and usually their pectoral fins are so expanded they remind us of wings. The skates undulate these fins to drive themselves forward, while the rays flap them like wings, flying through the water. In size these fishes range from monsters twenty feet across the wings to those less than one foot in width. So far as we know, the skates lay eggs with horny cases, while the rays have living young. Scientists have named and described several hundred species.
The weird-looking chimeras are at home in the deep waters of the oceans, where they are only rarely seen. These blunt-nosed fishes attain lengths of roughly three feet. They lay large eggs with horny capsules.
The sharks, skates, rays, and chimeras are different from most fishes in a number of odd and interesting ways. For one thing, all of them have gristle instead of true bone in their skeletons. They have jaws, two sets of paired fins, and a curious covering of spiny scales which gives the skin a texture like the surface of a file. Known as placoid scales, these are found only on fishes of this group.
These creatures also lack the true gill covers of the bony fishes; instead, the sharks, skates, and rays have from five to seven pairs of gill clefts, each opening to the outside, while the chimeras have a single opening on each side, formed by a fold of the skin – not a bone-supported, muscle-controlled organ like the gill covers of almost all bony fish. Sharks, skates, rays, and chimeras differ from the bony fishes in several other fundamental ways, and are so distinct that some zoologists do not consider them to be fishes at all. We place them in a class by themselves – the Chondrichthyes, a name meaning “gristle fishes.”