The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

are turned into each other. The creature moves by the contraction of the outer layer, which slowly squeezes in at the sides and back and forces the inner layer forward, so that as we watch the granules may be seen to stream along in the direction of the arrows in the figure. When it gets to the front end, the inner layer becomes converted into the outer layer, and when it gets back the outer layer is absorbed into the inner layer, so that as the animal proceeds it is constantly being urged forward by the contracting outer layer. It crawls, so to speak, by being everlastingly squeezed out of its skin. Its method of feeding is as elementary as its locomotion ; it consumes microscopic plants, other creatures like itself, and so on,

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Fig. 113. A Limax Ameba—one of the smallest and

simplest animals, built like a single cell.

and, lacking a definite mouth, it takes them in indiscriminately at any point on its surface ‘ it simply flows over them and round them and, thus embracing them in three dimensions, gets them into its own body. And when it has exhausted their nutritive possibilities it flows away from them, abandoning them from any point at the surface of its body. The individual in our drawing is prowling about among a number of crescentshaped microscopic plants; it has recently consumed three of them, and is dragging them along in its hinder part and slowly digesting them.

There are numerous species of amoebe, living for the most part in fresh- or sea-water or in damp soil. Some of them are parasitic, living in our bowels (which seem to have an irresistible attraction for microscopic organisms, for they are populated by a great variety 168

THE SCIENCE OF LIFE

CHAPTER 6

of different kinds), and although most of the parasitic amcebe are harmless, there is one species that causes a serious form of dysentery. The ameceba represented in the drawing belongs to a kind found in ponds ; itis called a “ limax amoeba ” because it crawls smoothly and steadily forwards in one direction like a slug (limax is Latin for slug). The “proteus amoeba ” has an even less definite shape ; its body-jelly can flow out into lobes at any point over the surface, and by throwing out a lobe and then flowing forwards into it, so to speak, the creature moves from place to place.

In this liquidly gelatinous body two definite structures are suspended. The first is an obvious, transparent sphere, situated near the hinder end of the body—a hollow sphere, filled with a watery fluid. If it is carefully watched, this sphere can be seen to grow slowly larger and larger until ultimately it bursts like a bubble, squirting its contents through the clear skin into the outer world. Then, in a little while, a tiny drop reappears, grows again, bursts again—and so on, with a rhythm of several minutes from burst to burst. This structure is called the contractile vacuole. There is an apparent parallel between its behaviour and the slow accumulation and roughly periodic expulsion of fluid that occur in the human bladder, and for a long time the contractile vacuole was regarded as a special excretory organ for getting rid of the waste-products of the creature’s activity. But, as a matter of fact, it is an organ for getting rid ofwater. The inside of the ameeba, like our own insides, contains salt and other dissolved substances, and these substances, by a simple physical process, are constantly sucking water in through the surface of the amceba from the outside. In technical language the dissolved salts set up an osmotic pressure inward. The water has to be everlastingly baled out again by the contractile vacuole, because otherwise the animal would swell up and burst. Most of the chemical interchanges between amoeba and its environment—the absorption of oxygen, the emission of carbon dioxide and other matters—occur indiscriminately over its whole surface and need no special organ.

The second structure is one we have already remarked in the tissue-cells of the higher types—the nucleus.

It completes a very striking resemblance