Frances Morgan, Age 10, Of Fredericton, N.B., Canada, for her question:
Can a shark grow new teeth?
The hungry shark is a toothy animal, and some of the big fellows are always
Eager to attack a swimmer. They can tear into human f1esh and bite off arms or legs. Some sharks have several rows of teeth, and when some are lost, there are always new ones ready to replace them.
The sea was infested with sharks long before the true fishes arrived. Their family tree dates back more than 300 million years, and they have never developed many of the features we find in the real fishes. A shark, for example, has no bones. Its skeleton is made of gristly cartilage.
The shark has other features which set him apart from other animals. And perhaps his most outstanding feature is his teeth. He has several rows of teeth in his mouth, and the most amazing fact is that his whole body is covered with spiky scales which resemble small teeth.
The teeth in the shark's mouth are shaped like little wedges. They are embedded not too firmly in the flesh of the ,jaw. Since they have no firm roots, the shark is forever losing whole sections of his dental equipment. But this is no problem at all.
A shark may have many rows of these rickety teeth, and. New teeth are always in the process of growing. When old teeth fall out or get stuck in a mouthful. Of food, new ones move in from behind to replace them. The teeth are splendid for biting off sections of live meat. But they are no good for chewing, and the shark swallows each mouthful in a gulp.
The toothy objects on the skin of the shark are often mistaken for scales. But they are unlike the scales of the true fishes. They do not overlap each other, and they do not grow in graceful curves. They are widely spaced and arranged in neat diagonal rows.
The experts call them placoid scales, or denticles. Each is shaped like a small winged dart, but it is made from hard material and coated like a tooth with enamel. The roots of the placoid scales are embedded deep in the shark's spongy, grey skin and fed by nerves and blood vessels.
The toothy tale of the ask is one of the wonders of nature. For the shark may we11 be the first animal to develop teeth of any kind. And, it seem, he developed teeth all over his body. Near his mouth, the placoid scales change to resemble the teeth in the shark's jaws. Teeth and toothy scales follow the same patterns of growth and seem to be related. And the worn or lost ones are soon replaced.