Brian Gough, age 11, of Salt Lake City, Utah, for his question:
How does a worm breathe?
The earthworm requires a steady supply of oxygen, like the rest of us. However he solves this basic problem in his own fascinating way. His rather featureless face has a mouth but no definite eyes. It also is without the nose that most of us rate as an essential organ in the breathing system. The earthworm has no nose and no lungs. Yet he successfully absorbs oxygen and his circulatory system relays it throughout his body. In the matter of circulation he is one up on us. He has more than one heart.
The land animals, we are told, gradually adapted their bodies to cope with life outside the water. Most of them grew tough skins to protect their inner tissues from brilliant light and the drying effects of the air. However, their original thin clammy skins could absorb dissolved gases directly into their surface capillaries. Thicker skins could not do this. So sometime_in.the.dim past, noses and lungs were developed as essential. pa>=ts ~af anew system for breathing air on the dry land.
The ancestors of the earthworm, however, ignored these newfangled changes. They kept their thin, moist skins and merely adapted their life style to avoid the problems of dazzling daylight and drying air. The modern earthworm has no reason to renounce his ancestral breathing system. The operation is carried on by his skin, working in co¬coperation with a circuit of blood vessels powered by five pairs of simplified hearts.
As in the rest of us, the earthworm's vital systems of respiration and circulation are mainly automatic. However, we have a duty to provide our lungs with available supplies of fresh air. The earthworm has a duty to keep his thin clammy skin in.moist, shady surroundings. This he does by burrowing below ground, where the cool air has less drying power. He also arranges his life to avoid the bright light of day. The rest is up to his ancestral skin.
The oxygen of the air has a strong tendency to dissolve in moisture. The earthworm keeps his skin moist and adds clammy material from mucous glands, situated mostly in the segmented grooves at his head and tail ends. Molecules of oxygen dissolve in this clammy film. The surface skin is so thin that gaseous molecules can pass through, much as the gases we breathe pass through thin surface membranes in our spongy lungs. Just below his skin, networks of tiny capillaries eagerly absorb molecules of dissolved oxygen and return waste gases to the air.
The branching capillaries connect with a main blood vessel that runs from one end of the worm's body to the other. It has transverse pairs and other vessels branching out to the digestive system, other organs and living tissues Five pairs of the transverse vessels are enlarged and fitted with muscles. They operate as hearts to pump the blood through the circuit.
The earthworm's sense organs are equally ancestral. His invisible eyes are numerous light sensitive cells, most of them near his head and tail ends. He also has numerous small organs that are sensitive to touch, temperature and maybe chemicals. All of these sense organs are located in his very remarkable skin.