Dale Steven Robinson, age 13, of Asheville, N.C., for his question:

HOW DO FISHES DIGEST BONES?

The average fish, as we know, is full of sharp little bones, and horrible things happen when we swallow one by accident. Yet this same fish dined mostly on other bony fishes, plus crusty mollusks. Surely he must have special internal organs designed to cope with these tough digestive problems on his daily menu. And so he has.

The 20,000 or so different fishes come in a vast variety of sizes and fantastic shapes. Most of them are hungry meat eaters who gulp down fishes smaller than themselves, bones and all. A few species feed on plankton and floating seaweed, and a few are scavengers who clean up decaying plant and animal food.

One would expect the plant eaters to make do with a digestive system suitable for coping with fairly soft foods. However, some of these types have a muscular gizzard suitable for grinding up tough fibers. As a rule, the fish eating fishes have no gizzard. Yet somehow they manage to cope with bones and the tough shells of crusty mollusks. This is possible because all parts of the digestive system are tough and strong, and some parts are stretchable.

The average fish has a mouthful of sharp teeth, anchored firmly in his jaws. He has more teeth in his throat and maybe more on his tongue or on the roof of his mouth. However, he does not use them for chewing. They are for biting off large, swallowable helpings, which go down his toothy throat to his stretchable gullet.

Sometimes the larger bones and tough shells are crushed or partly crushed in the muscular throat and gullet. Otherwise the whole mouthful passes to the stomach. Here it is mashed to a pulp and various juices are added to extract some of the usable food elements.

From the stomach, the mixture of crushed bones and partly digested pulp passes to the intestines. There more juices are added to complete digestion. The undigested wastes, including bits of bone, pass out through the anus, which is on the underside in front of the last lower fin.

Actually the digestive system of a fish eating fish is not very different from the human digestive system. However, obviously it must be made of tougher material with extra strong internal walls  otherwise his insides would be pierced and scraped by sharp bones and shells. What's more, his stretchable gullet helps to protect him from bits of brittle bone.