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same all round; there is nothing that we can regard as a head, and their bodies have no right or left, but we find among them primitive sense organs. Many are soft-bodied animals which could leave no fossil traces except impressions they have made on a muddy shore, but coral fossils are not only found in great numbers, but actually form the substance of many rocks.

The remaining groups of the backboneless animals have all evolved nerves, blood-vessels, and a digestive system which is shut off from the main body cavity.

The 'sea-urchin, starfish, and sea-lily are living representatives of classes all of which have left fossils, and other ancient fossils represent related classes now extinct. They are important as rock-builders. These animals, though they mark a distinct advance in organization, are still the same all round. Nothing resembling a head has yet been evolved.

Sea-worms, earth-worms, water-worms, and leeches have segmented bodies and were probably the first animals which began habitually to move with one part of the body always in front—in other words, to acquire a head, and consequently a right and left. The head, being the portion with which new ground would be first explored, would have its nerve cells stimulated, says Professor J. A. Thomson, and thus the evolution of the brain might begin. Darwin found that worms showed distinct intelligence in their way of drawing leaves into their burrows by the narrow ends so as to have as little trouble as possible.

Insects, centipedes, lobsters, barnacles, spiders, scorpions, and many less familiar animals form the next great group.

They have a segmented body produced into hollow, usually jointed limbs. In the more primitive forms the whole body is divided into similar segments, and a pair of limbs is borne on each, but in later forms several segments are fused more or less completely, especially at the head end of the body. A more important development is that, though in structure all these limbs are organs of locomotion, some at the front end of the body round the mouth are used for seizing and biting

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food. Feet have become jaws. In water animals of this group some limbs behind the jaw limbs have developed plates which act as gills. The land animals breathe by means of lung sacs or tubes opening to the air. In fossil scorpions of the carboniferous age we see the conversion of gills into lungs as the animal emerged from the water. All existing scorpions

are land dwellers. The development of eyes and feelers in animals of this group marks another stage in the evolution of the head. All the classes are represented by numerous fossils. Insects are the most highly organized animals of the group and are of great importance on account of their connexion with the evolution of the flowering plants.

Molluscs, including oysters, snails, whelks, cockles, and cuttlefish, are among the more highly organized backboneless animals, having a distinct head, a mouth, digestive, and nervous systems, blood vessels, heart, and gills. Their soft bodies have no internal skeleton, but a hard shell is usually formed on a part of the outer surface, and these shells form the chief part of their fossil remains. The cuttlefish, squid, and octopus have long arms or tentacles round the mouth, strong jaws, power of locomotion — all modifications of structure which would enable them to prey on other animals and conspicuous eyes, which are as highly developed as those of a backboned animal.

It has been noted as one of the main distinguishing marks between plant and animal life that animals in general are much more active than plants, the reason being that plants, obtaining their food from the air and the soil, have no need of locomotion to seek it, while this power is usually a necessity to animals, since they live either on plants or on other animals. Plants, however, have a limited power of movement, and this is all that is possessed by some of the lower backboneless animals. The simplest animal organisms of all, the protozoa, have some locomotive powers, but sponges, some starfish, sea lilies, and corals for nearly their whole lives, do not actively change their position, or are even anchored to something, such as a shell, other corals, or the sea floor. Movement is confined to arms and to vibrating hair-like organs which collect food and regulate the process of breathing. The higher the organization of plants and animals, however, the less they resemble one another; and we find in animals that the progress in locomotion, which distinguishes them more from plants, is attended by development of the nervous and muscular

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systems and of the brain. Movement is easier on water than on land, where balance and weight assume a new importance, and we find the animals that have adapted themselves to dry land more highly organized than their brethren of the water. In the group that includes insects, centipedes, lobsters, and spiders the most highly organized animals are the flying insects, which have adapted themselves not only to land but to air. When we come to backboned animals we find that the great age of the reptiles was that in which they had representatives at home in air, on land, and in water, and we recognize at once that the highest animals the birds and mammals—are those that display the greatest and most continuous activity.

CHAPTER IX

BACKBONED ANIMALS: FISHES, AMPHIBIANS,

REPTILES, AND BIRDS

N immeasurably ancient rocks, which have given us little but seaweeds to tell us of the plants that

were living when they were laid down, appear the remains of the first backboned fish. There are immensely older rocks containing fossils, but among these no fossil fish of any description are to be found. We must not suppose on that account that none were in existence before the first evidence of them is found in the rocks. Before the strange fish forms which have left remains in the shape of teeth or scales or internal skeletons there were probably entirely soft-bodied fishes, which could leave no record of their lives for later ages to read. The earliest backboned fishes probably resembled the sharks and dog-fish of the present day; they have left little but teeth and spines, but from these we know that in later epochs their descendants sometimes reached a huge size, probably as much as a hundred feet in length. With the exception of these ancient sharks few of the earliest fishes have present-day representatives. Fish with thin, flexible scales and symmetrical tails, such as carp, roach, salmon, and herring, as well as all flat fish, are of comparatively recent origin, making their appearance in the chalk rocks that saw the first flowering plants. The contemporaries of the ancient sharks were fishes with hard, bony scales, large bony plates on the head, and

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