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

FURTHER PATTERNS OF INDIVIDUALIZED ANIMAL LIFE

adult man or woman—except that in the case of echinoderms the organization and symmetry are very different in child and adult.

§ 4 Segmented Worms

The next invertebrate phylum, the Annelida, includes such animals as the earthworms, the leeches, and the lugworms—a series of animals which are distinctly like the arthropods in their organization. They have, however, no rigid armouring, and their blood does not fill their body-cavities but is confined within definite vessels. It may be red as ours is, or it may be green, but it is never coloured pale blue by hemocyanin. In these respects they differ from arthropods. But they have a central nervous system running along their bellies, and a long, rhythmically contracting artery that plays the part of a heart running along their backs ; in these matters, and in the way the nervous system forms a loop round the mouth, they are like arthropods. Moreover, they are segmented. We noted that the body of any arthropod may be regarded as a modified chain of segments, each of which bears one pair of appendages. This type of structure is even more obvious in annelids. The body shows a series of ring-like grooves that divides it into segments; on opening it up we should find that the body-cavity is divided into a series of compartments by transverse partitions corresponding to the rings. Each of these segments contains a pair of tubes that act like kidneys, each contains a ganglionic swelling of the nervous system, and in many forms each contains a pair of testes or ovaries. Moreover, in all annelids except the earthworms and leeches, each segment bears a pair of appendages, not jointed but bearing tufts of bristles, and used in most species as primitive limbs, which strengthens their resemblance to the segments of arthropods. If the reader will glance back at our description of Peripatus he will probably be disposed to agree with the many authorities who regard the arthropods as a sort of higher, more complex, development of the annelid plan.

There are three main groups of annelids. The first includes the burrowing earthworms and one or two similar but aquatic forms ; one species, from Tasmania, attains a length of six feet. Charles Darwin’s book on Vegetables Moulds and Earthworms has shown what a vitally important part is played by earthworms in loosening and turning over the soil. The animals consume earth in

great quantities and digest nutritious matter out of it as it passes down their intestines ; they defecate it on the surface of the ground as ‘“‘ worm-castings.” In rich garden soil, where worms are especially numerous, there may be over 50,000 individuals to an acre ; in these circumstances more than ten tons of earth will pass through their bodies and be brought to the surface in one year, and in ten years this will form a continuous layer of finely divided surface soil at least two inches deep. In Darwin’s own words, “ The plough is one of the most ancient and most valuable of man’s inventions ; but long before he existed the land was, in fact, regularly ploughed and still continues to be thus ploughed by earthworms. It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organized creatures.”

Earthworms, we may note, are “ hermaphrodite,” male and female in one, and their embraces are reciprocal. This is not by any means the only departure from the relentless division of individuals into two sexes that plays so important a part in our own social life. Snails, for example, are both male and female, but in this case the two faculties are not exercised simultaneously ; the creature takes turns, being now one sex and now the other. The edible oyster alternates slowly but regularly between male and female, and there are many cases in the invertebrates of animals which normally start their lives as males, become mature, and then at a certain age turn over and become female for the rest of their lives. The vertebrate way is not the only way. Indeed, as we proceed with our survey we shall find increasingly striking departures from our own way of life, even in those matters which we regard as the very warp and woof of it.

The second group of annelids is of less economic importance, but it includes a greater variety of forms, all of which are marine. It includes among its hundreds of species the graceful, sinuous paddle-worms of our shores, the iridescent sea-mouse, the lob-worm, which is often used as bait, and a number of animals which live in tubes attached to rocks, sea-weeds and the like, and feed on suspended matter, much as bivalves or sea-squirts do, by means of filtering crowns of delicate tentacles.

The third group comprises the leeches, terrestrial, fresh-water, and marine. They have suckers fore and aft, and the majority live by attaching themselves to other creatures and sucking blood. A few, however,

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