ph.

i

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m

or crawling. They also give support to a brain, which is in the form of a collar encircling the pharynx near the head, and to the great nerves which extend from it. Still further within the transparent body the alimentary canal may be distinguished as a straight tube passing directly through the animal. The alimentary canal lies freely in a great space, the body A cavity, traces of which may exist in the flatworms in the form of hollow spaces into which the kidneys open. It is possible that in this form also the kidneys open into this space, and it is roomy enough besides to afford lodgment for the reproductive organs in addition to a large amount of fluid which is probably somewhat of the nature of blood. A space in some respects similar to this occurs in all the animals above this group, and as we shall see, it is often curiously modified and serves for a number of different and highly important purposes. In the roundworms the fluid it contains probably acts in the nature of a blood system, distributing the food and oxygen to various parts of the body and carrying the wastes to the kidneys for removal.

FIG. 37. Thread- or round. worms. A, vinegar eel (Anguillula); m, mouth; ph., pharynx; i, intestine; ov.. developing young. B, Trichina. From Nature, greatly enlarged.

63. Multiplication. In the matter of the production of new individuals the greatest differences exist. In some threadworms, for example the "vinegar eel," eggs develop within the body and the young are born with the form of the parent. In other cases the eggs are laid in the water, where they, too, may directly grow to the adult condition; but in

the greater number of species the development is roundabout, and one or more hosts are inhabited before the young assume the adult condition. Such is the case with the dreaded Trichina (Fig. 37, B), which infests the bodies of several animals, particularly the rat. When these forms are introduced into the alimentary canal of the rat, for example, they soon lay a vast quantity of eggs, sometimes many millions, which develop into young that bore their way into the muscles of the body, where they may remain coiled up for years. If the body of the rat be eaten by some carnivorous animal, these excessively small young are liberated during the process of digestion and rapidly assume the adult condition in the alimentary canal, likewise giving rise to young which pursue again the same course of development.

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Another example of a complicated life history is in the Gordius or "horsehair snake (a true worm and not a snake) frequently seen in the spring in pools where it lays its eggs. These eggs develop into young which bore their way into different insect larvæ, which are in turn eaten by some spider or beetle, and the worm thus transferred to a new host. In this they grow to a considerable size, and then make their exit from the body of the host and finally become adult.

64. Spontaneous generation. —The ancients believed that many animals were spontaneously generated. The early naturalists thought that flies arose by spontaneous generation from the decaying matter of dead animals; from a dead horse come myriads of maggots which change into flesh flies. Frogs and many insects were thought to be generated spontaneously from mud. Eels were thought to arise from the slime rubbed from the skin of fishes. Aristotle, the Greek philosopher, who was the greatest of the ancient naturalists, expresses these beliefs in his books. It was not until the middle of the seventeenth centuryAristotle lived three hundred and fifty years before the

birth of Christ-that these beliefs were attacked and began to be given up. William Harvey, an English naturalist, declared that every animal comes from an egg, but that the egg might "proceed from parents or arise spontaneously or out of putrefaction." In the middle of the same century Redi proved that the maggots in decaying meat which produce the flesh flies develop from eggs laid on the meat by flies of the same kind. Other zoologists of this time were active in investigating the origin of new individuals. And all their discoveries tended to weaken the belief in the theory of spontaneous generation.

If some

Finally the adherents of this theory were forced to restrict their belief in spontaneous generation to the case of parasites and the animalcules of stagnant water. It was maintained that parasites arose spontaneously from the matter of the living animal in which they lay. Many parasites have so complicated and extraordinary a life history that it was only after long and careful study that the truth regarding their origin was discovered. No case of spontaneous generation among parasites is known. water in which there are apparently no living organisms, however minute, be allowed to stand for a few days, it will come to be swarming with microscopic plants and animals. Any organic liquid, exposed for a short time, becomes foul through the presence of innumerable bacteria, etc. But it has been certainly proved that these organisms are not spontaneously produced by the water or organic liquid. A few of them enter the water from the air, in which there are always greater or less numbers of spores of microscopic organisms. These spores germinate quickly and the rapid succession of generations soon gives rise to the hosts of bacteria and Protozoa which infest all standing water. If all the active organisms and inactive spores in a glass of water are killed by boiling the water, "sterilizing" it, as it is called, and this sterilized water be put into a sterilized glass, and this glass be so well closed that germs

or spores can not pass from the air without into the sterilized liquid, no living animals will ever appear in it. It is now known that flesh will not decay or liquids ferment except through the presence of living animals or plants.

To sum up, we may say that we know of no instance of the spontaneous generation of organisms, and that all the animals whose life history we know are produced from other animals of the same kind. "Omne vivum ex vivo," All life from life.

ANNELIDS OR SEGMENTED WORMS

65. The earthworms and their relatives.-Leaving the groups of the parasitic animals, which have been driven from the field of active existence and in many ways are degraded by such a mode of life, we pass on to the higher free-living worms, where brilliant colors, peculiar habits, or remarkable adaptations render them peculiarly interesting. In considering first their general organization, we may use the earth

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FIG. 39.-Earthworm (Lumbricus terrestris). m, mouth; c, girdle or clitellum.

worm (Fig. 39) (sometimes called angle-worm or fish-worm) as a type because of its almost universal distribution.

The body is cylindrical, shows well-marked dorsal and ventral surfaces, and, as in all of the annelids, is jointed, each joint being known as a segment. Anteriorly it tapers to a point, and the head region bearing the mouth is illdefined, unlike many sea forms, yet serves admirably for tunneling the soil in which all earthworms live. In this process the animal is also aided by bristles or seta which project from the body wall of almost every segment and may be stuck into the earth to afford a foothold.

66. Food and digestive system. The earthworms are nocturnal animals, seldom coming to the surface during the day except when forced to do so by the filling of their tunnels with water or when pursued by enemies. At night they usually emerge partially, keeping the posterior end of the body within the burrow, and thus they scour the surrounding areas for food, which they appear, in some cases at least, to locate by a feeble sense of smell. They also frequently extend their habitations, and in so doing swallow enormous quantities of earth from which they digest out any nutritive substances, leaving the indigestible matter in