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

BOOK 2

up into cells, consists of an uninterrupted sheet of protoplasm freckled with nuclei. We may, however, note that these creatures are not all parasitic. A huge number of them live harmlessly in salt or fresh water or in soil or the tissues of rotting plants. Some species are free when they are young, but become parasitic when they mature—such as Strongylus and Dochmius, which infect man, and Tylenchus scandens, which invades the tissues of cereals. In some, on the other hand, the young are parasitic while the adults are free-living ; an example of this is afforded by Spherulia bombi, whose young grow in the tissues of bees and wasps. Bana lly, many, as we have seen, are parasitic throughout, although the larve and adults may infect different hosts or different parts of the same host; and many of these are very troublesome indeed to man.

Mouth

§ 6 Fig.92.Plan- —— Flatworms aria, a_ kind The Platy-

of flatworm,

showing the digestive tubes in black (magnified about six times) .

helminthes or flatworms are for the most part parasitic. Their organization is curious, and we may introduce ourselves to it by examining a particular member of the group.

Planaria lactea is a white, flat, worm-like creature, about three-quarters of an inch long, that leads the life of a scavenger at the bottom of fresh-water ponds, nourishing itself for the most part on dead organic matter. Usually it travels along with a smooth gliding motion ; from time to time it varies this somewhat monotonous method of progression by “looping” like a looper caterpillar.

The head of Planaria is interesting. Our own heads are chiefly remarkable for two

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THE SCIENCE OF LIFE

CHAPTER 3

reasons ; first, because they contain the chief sense-organs and the brain, and second, because they contain the mouth and the apparatus for taking in food. Now the head of Planaria is only half a head ; it carries the brain and eyes, but it has no mouth. The mouth, instead of being at the front end of the body, is placed at the end of a long tube that sticks out of the middle of the belly ; moreover it combines the functions of mouth and anus.

Another curious feature of this animal is that it possesses no circulatory system at all, although, unlike an intestinal roundworm, itis an actively moving creature. The bodycavity is filled with spongy connective tissue, through which the digestive and excretory systems ramify like the branches of trees. Since there is no blood, the branching intestine has itself to carry food into every corner of the body and the branching kidneysystem has to run into every corner and drain the waste-products away. Because of this fact the anatomy of Planaria looks very complicated ; if, in the accompanying figure of the digestive apparatus, the reader imagines an equally arborescent system of fine excretory canals, imiricate reproductive organs (both male and female, in the same body), and a web-like nervous system he will get some idea of the labyrinthine nature of this tiny inside. But the complexity is more apparent than real. The organs are elaborately branched, but there are not nearly so many kinds of organs, or so many kinds of tissues, as in a vertebrate.

Moreover, Planaria has no special respiratory organs ; its body is so thin that enough oxygen to satisfy its needs can diffuse in through the tissues.

The phylum platyhelminthes comprise three classes. ‘The first class is that to which Planaria belongs ; they live for the most part independently in damp places on the land, in fresh water, and in the sea, but one or two of them are parasites.

The second class is the flukes—a group consisting entirely of parasitic forms. ‘These animals show the most ingenious devices for invading and destroying their unwilling hosts; we may illustrate the point by following the life-history of the liver-fluke, Fasciola hepatica, found in sheep and causing the wasting disease known as sheep-rot, particularly in beasts that feed in wet, soggy pastures—a life-history that is surprising because it involves a regularly alternating rhythm of several quite different generations.

The flukes that inhabit and devour the liver of an infected sheep are hermaphrodite