Michael Hall, age 11, of Spokane, Wash., for his question:

What is a cephalopod?

The word means head footed and the average cephalopod seems to be all head and trailing tentacles. The family, Cephalopoda, is an ancient one. There were cephalopods in the early seas 400 million years ago and there have been cephalopods in the deep oceans and the tidal waters ever since. These animals form a class of the Mollusca phylum which includes the snails, the oysters, mussels and clams. Apart from one member, however, the cephalopods are without shells.

The shell dwelling cephalopod is the pearly nautilus of tropical seas. His delicate shell is a coil of chambers, each one bigger than the last, which is the one in which he now lives. The paper nautilus, known as the argonaut of Mediterranean waters, only seems to live in a shell. The papery shell seen scudding over the waves is actually the nest in which the eggs are cradled.

But there are better known members of Cephalopoda   and all of them are astounding to us. They range from a few inches to mighty giants and one is the biggest of all the backboneless animals. All have huge heads and staring eyes. Some have a monstrous parrot beak. Most have eight or ten tentacles though some have as many as 90. By now you have guessed that a large part of the Cephalopoda class is made up of the squids and the octopuses.

The octopus has eight arms, each studded with a double row of button suckers. These trailing arms are arranged around the mouth, which is a round hole fitted with powerful jaws and a sawlike tongue. The powerful beak is set inside the circular mouth opening. The tentacles fan out from the base of the big head like a constantly moving collar.

Some octopuses are ten feet long and weigh    70 pounds. Most are small, shy creatures living in shallow seas.

The squid is shaped like an arrow and is almost as fast. He has ten arms, one pair much longer and stronger than the rest. Many of the squids are small creatures no longer than a few inches and schools of them travel together in shallow waters. But the granddaddy of the squid clan keeps to the deep ocean and is rarely seen. He is the giant squid who may tip the scales at half a ton. His body may be 20 feet long and his arms 35 feet long. On his rare visits to the surface, this giant of the deep has been mistaken for a sea serpent.

All the cephalopods are ocean dwellers who take their oxygen from the water through gills. They travel by taking in water and squirting it out. In fact, the cephalopods have been using jet propulsion for some 400 million years. All but the nautilus and the argonaut are able to squirt a dye into the water and hide behind its inky cloud.