Diana Kelly, age 12, of Williamsport, Pennsylvania, for her question:

Do snakes have bones?

If you have watched a snake move, you know that he slithers over the ground with the graceful, waving motion of liquid water. He seems to have not a bone in his pliable body, but nothing could be farther from the truth. He happens to be just about the boniest creature on the earth. True, he has no toe bones or finger bones. He has no visible limbs that require bones. But his ancestors had legs and remnants of hip and thigh bones may be found buried inside the bodies of certain snakes. Every snake, however, excels in jaw bones. Actually he has two for one, because his jaw bone is divided in two separate sections and linked with an elastic band of non¬bony cartilage.

When it comes to the number of ribs, the snake is a champ. His supple body may be encased in a tunnel of 400 or more pairs of bony ribs, and each pair is attached to a vertebra of his pliable spine. He has extra vertebrae in the tail section ex¬tending below his ribs. The snake's abundant supply of bony ribs makes it possible for him to slither along as though his body had no skeleton inside it at all. He moves by stretching and contracting sections of his ribs on alternate sides. The rubbery waves occur as the alternate sections of ribs spread apart and squeeze together.