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

BOOK 2

forming the chief food of a host of crustaceans, young fish and other larger creatures and thus indirectly nourishing man himself. The diffuse phosphorescence of the sea is due largely to swarms of flagellates. Moreover, there are kinds of flagellate which live parasitically in our bodies or the bodies of our cattle and cause grave tropical diseases such as the sleeping-sicknesses of Africa, and the Indian scourge kala-azar. Theoretically they are interesting because some of them are coloured green with chlorophyll and lie on the border-line between animal and plant, and because others combine together to form little colonies, a link between the independence of amceba and the communal life of our own body-cells.

A third class are the Sporozoa. They are all parasites. The most troublesome kinds are those of the various mosquito-borne fevers and agues, ranging from the comparatively mild intermittent fevers, which have been abolished by the drainage of our own fen and marsh districts, to the deadly malarias of tropical countries.

The fourth class, the Jnfusoria, includes some of the largest and most elaborately organized protozoa. They have definite mouths ; their bodies are covered with hard pellicles ; instead of one nucleus they have two, of different kinds, one for the chemical business of every day, the other reserved for sexual union and other rare reproductive occasions. Instead of one or two propulsive whips, like the whips of flagellates, they possess whole batteries of vibrating hairs or cilia by means of which they swim. The cilia may be of various shapes and sizes, and are variously arranged on the body, and the modes of progression of their owners are correspondingly diverse. Some infusorians glide more or less smoothly through the water ; some crawl and then dart; some spring by means of catapult-like cilia at their hinder ends. Among the commoner freshwater forms well known to microscopists are Paramecium, the slipper animalcule—quite large for a protozoon, nearly a hundredth of an inch in length and visible as a tiny speck to the naked eye—Vorticella, the bellanimalcule, a graceful living goblet fastened to some solid mooring by a long stalk, which can shorten to a tight spiral when the animal is alarmed and so pull it back out of dangerand a host of others. Attractive though the infusoria are to the possessor of a microscope, they are of little direct importance to man. They do him no good, and, except for one species that lives in the intestines and causes dysentery, they do him no harm.

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THE SCIENCE OF LIFE

CHAPTER 6

§ 3

Plant-animals and Seaweeds

Most of the protozoa are quite obviously animals ; they wander about and feed in the animal manner. But there are curious exceptions. There is, for example, a tiny creature known as Euglena, one of the flagellates possessed of a lashing whip by means of which it swims about. Now Euglena has got two perfectly distinct ways of nourishing itself and growing. It has a mouth and can take things into its body and there digest them after the manner of animals. But it is also green in colour ; it possesses chlorophyll and can build up its tissues out of the simplest ingredients after the manner of plants. It can do whichever it likes. Here then is a problem: is Euglena an animal or a plant ?

We noted in the last chapter that chlorophyll is the characteristic feature of vegetable organization. It enables plants to dispense with the complex food that animals require, and a number of the most striking features in plant anatomy depend simply upon that fact. It accounts, for example, for their lack of muscles and nerves. But it is not an absolute criterion; there are organisms, such as the fungi, which are more like typical plants than animals in their organization, but which do not possess chlorophyll. Indeed, it will become more and more apparent as we proceed with this chapter that there is no absolute criterion. ‘Typical plants such as buttercups and oaks and cabbages differ strikingly enough from typical animals such as mice and men and lobsters; they are green and feed on substances of simple constitution ; they do not move about and apparently they do not feel ; their cells are enclosed in tough boxes of cellulose. And as long as we confine ourselves to the more obvious organisms the line is easy enough to draw. But when we peer through our microscopes at the curious world of microorganisms we find the distinction breaking down. We find, for example, that there are colourless flagellates which whip about and feed like animals; we find very similar flagellates, whipping about in much the same way but green and building up their substance in the manner of plants. We find some of the filamentous algz, delicate green threads of protoplasm, motionless and nourishing themselves like plants; we find the moulds, which have very similar threads but colourless, motionless also but nourishing themselves in darkness on substances of complex constitution, And we find other fila-