Jet-Propelled Shellfish

"I can remember when the straight-shelled cephalopods were quite important
 in the prehistoric seas. Now they are nothing but fossils."

      Now that you are listening to my story, I know that you will realize how important I am. But many people have ignored me as I lie here because they don't realize what wonderful adventures a pebble can have. Once, I did almost get picked up by a lively little boy; but just as he reached down to get me, his attention was drawn to something else. Before I could make him hear me, he had snatched the object up and was running quickly off, calling to his friends to come and see what he had found. Oh, I suppose he had reason to be excited, for I had caught a glimpse of his prize. It was part of a cephalopod shell.

      I can remember when the straight-shelled cephalopods were quite important in the prehistoric seas. Now they are nothing but fossils while I am still my same old self, except that I am a little smaller now because of the trying ordeals of my long career.

      The cephalopods were members of the same family as the modern squid and the octopus. Cephalopods, however, had shells that were coiled, partly coiled, or straight. They were the biggest shellfish that ever lived on the earth. The shells of some cephalopods stuck straight out behind them like long narrow ice cream cones, and others were only as big as pipe-stems. But many cephalopods had shells almost twenty feet long and a foot wide. They were the masters of the sea, and they were flesh eaters.

      The word cephalopod comes from the two Greek words "head" and "foot." The foot, which was

divided into a number of arms, was wrapped around the head. The foot was used by the cephalopod in moving about, but it was not used in the same way that a foot is usually used. By taking in water and then squirting a jet of it out of a funnel under his head, the cephalopod was able to shoot through the water like a torpedo. This method of moving was similar to the way in which the fastest planes move today, for the cephalopod was traveling by means of jet propulsion. The only thing wrong with his method of traveling was that he went through the water backwards.

      Just imagine the silly cephalopod not only traveling through the water in reverse but also carrying with him that long straight shell. Later, the shells of these creatures were curved. First they were only loosely coiled, but in time they coiled very tightly. Then the cephalopod was much better off because the coiled shell was not so cumbersome as the long straight one. Then, too, the air-filled compartments of the big shell were right over his body. This made him better balanced and he could float more easily. Some of these curved shellfish by Cretaceous time had shells almost seven feet across. They hold a record for size that has not been equaled since.

      Eventually all the cephalopods with straight shells died out. Perhaps competition with animals that had backbones proved too much for them. Their long shells, however, were fossilized and left a record in stone of their existence. The modern squid the descendant of the cephalopod depends for protection not on a heavy shell but on his ability to move quickly through the water when there is danger. A small, useless, horny plate, shaped like a feather and buried in the flesh of the squid, is all that is left of the great big shell of his ancestors. Just think how much luckier he is than his extinct cephalopod relatives.