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

THE LOWLY AND MINUTE

devoured, but the survivors can multiply so rapidly that the supply is inexhaustible. And the entry of a few parasitic protozoa into the blood of a man may fill him in a very few days with pullulating millions of invisible enemies—enough to interfere seriously with his vital processes, perhaps enough to destroy him.

There is another important aspect of the

curious methods of multiplication that are found among these tiny creatures, another reason why they can increase so rapidly. We have noted that the individuals are very small and that they do not survive for long. But let us consider again the way in which they cease to be. A protozoon ends its individual existence by dividing its own body into living bits—into two halves, or into a greater number of fragments. Every one of these bits grows into a complete new individual. Try to imagine that in terms of human experience. Imagine that when a man or a woman reaches a ripe old age he or she simply divides into two halves, each of which is a growing child! If all goes well with a protozoon its tissue never dies ; every bit of its body is handed on for the use of future generations. Our own method is very different from that. Little bits of our bodies do indeed live on and grow to be our heirs, but most of our substance dies when we as individuals cease to be. With every generation there is an apparent wastage, a scrapping of great quantities of material. In the protozoa, on the other hand, there are no corpses; every bit of protozoan tissue continues to multiply and grow. It is potentially immortal ; although the individuals exist only for an hour or so, their tissue lives for ever. _ The protozoa, then, put their whole weight into the business of reproduction. They do not relegate that all-important task to a small part of their bodies. The method that we have just discussed is a sexless one, but we may pause for a moment to consider the sexual methods that they sometimes employ, for in this they are equally whole-hearted. The details of sexual reproduction vary in the different kinds of protozoa. But the essence of the process is nearly always this : a complete mixing together of the substance of two originally separate individuals. They meet and come together in pairs, and they blend as completely as drops of water run together. They achieve an intimacy of union that we can never aspire to, becoming in a perfectly literal sense one flesh. And the resulting blend lives on as a single thing, after the manner of protozoa.

These protozoa are divided into four classes. There are first the Rhizopoda, which include amceba ; their name means “ rootfooted,” and is an allusion to their power of everlastingly changing their shape by protruding and withdrawing blunt lobes or fine branching threads of living substance. The other kinds of protozoa are more definitely and permanently shaped. Two orders of this class are important—the Foraminifera which build shells of carbonate of lime and the Radiolaria which build flinty shells of silica. When these creatures multiply in the protozoan way by division of their bodies into a family of children, their offspring abandon the parental shell and make new ones of their own. All the Radiolarians and many of the Foraminiferans live floating in the waters of the sea. There is consequently a perpetual soft rain of abandoned shells dropping on the ocean floor. The creatures are incredibly numerous and they can multiply rapidly, so it will be manifest that infinitesimal though they are their accumulations may be immense. In some places the sea-floor is covered deeply with a greyish mud called “ globigerina ooze,” which consists largely of billions and billions of the abandoned shells of a foraminiferan called Globigerina. In the deep sea, where the physical conditions are such that abandoned limy shells are re-dissolved in the water, only the siliceous shells of the radiolaria remain and form a deposit called “ radiolarian ooze.’ In the course of past ages such oozes have hardened into earth and rocks, and as, during the evolution of our globe, the sea has advanced and receded over most of its surface, they have been left as geological strata on dry land. Huge thicknesses of white chalk and limestone consist very largely of the microscopic skeletons of foraminifera. The rock of which the Pyramids of Egypt are built is composed largely of the shells of Eocene foraminiferaand the earth known as “ tripoli stone,” used commercially as an abrasive or an absorbent, is composed mainly of radiolarian skeletons, the variety found in the Barbados containing at least four hundred different species.

In a second class of the protozoa, the Flagellata, the individuals are characterized by the possession of one to four long lashing tails or flagella (singular, flagellum), and flog themselves along through water much as spermatozoa do. The group 1s a large one and profoundly important, both from the practical and the theoretical point of view. ‘They are of practical importance because they swarm in incredible multitudes in the sea,

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