upon which they rest. In the skin are embedded multitudes of small spherical sacs filled with pigments of various colors, chiefly shades of red, brown, and blue, each sac being connected with a nerve and a series of delicate muscles. If the animal settles upon a red surface, for example, a nerve impulse is sent to each of the hundreds of color sacs of corresponding shade, causing the muscles to contract and flatten the bag like a coin, and thus exposing a far greater surface than before, they give the animal a reddish hue. In the twinkling of an eye they may completely change to another tint, or present a mottled look, and some may even throw the surface of the skin into numerous small projections that make the animal appear part of the rock upon which it rests. These devices not only serve for protection, but they also aid in enabling these mollusks to steal upon their prey, chiefly fishes, which they destroy in great numbers with lionlike ferocity.

The devil-fishes and a number of other species are usually found creeping along the sea bottom, generally near shore, and are solitary in their habits, while the squids remain near the surface and frequently travel in great companies, sometimes numbering hundreds of thousands. In size they usually range from a few inches to a foot or two in length, but a few devil-fishes and squids attain a greater size, some of the latter reaching the enormous length of from forty to sixty feet. There are many stories of their great strength and of their voluntarily attacking people and even overturning boats, but the latter are in almost every case sailors' yarns.

In their external organization the cephalopods have little to remind one of any of the preceding mollusks, and their internal structure shows only a distant resemblance. In the Octopi (Fig. 52) the shell is lacking; in the squid it is called the pen, and consists of a horn-like substance without any lime deposit; in the cuttlefishes it is spongy and plate-like, and is a familiar object in the shops; and, finally,

in the nautilus it is coiled and of considerable size, and, unlike that of any other cephalopod, it is carried on the outside of the animal. Interiorly it is divided by a number of partitions into chambers, the last one of which is occupied by the animal.

The alimentary canal shows some resemblance to that of other mollusks, but, as in the case of the other systems of the body, it possesses a far higher state of development. The mouth is situated in the center of a circle of arms, which in reality are modified portions of the foot, and is furnished with two parrot-like jaws. From this point the esophagus leads back into the body mass to the stomach, which with the liver and intestine are sufficiently like those of the clam and snail to require no further comment.

Respiration is effected by the skin to a certain extent, but chiefly by two gills (four in the nautilus), and the circulatory system, which conveys the blood to and from these organs and over the body with its complex heart, arteries, capillaries, and veins, is more highly developed than in any other invertebrate.

As might be expected in animals with so great sagacity and cunning, the nervous system and the sense-organs reach a degree of development but little short of what we find in some of the vertebrates. The chief part of the nervous system is located in the head, protected by a cartilaginous skull, a very rare structure among invertebrates; and while the different ganglia may be recognized in a general way and be found to correspond to a certain extent to those of foregoing mollusks, they are so largely developed and massed together that it is impossible at present to understand them fully. From this point nerves pass to all regions of the body, to the powerful muscles, the viscera, and the organs of special sense, controlling the complex mechanism in all its workings.

There is no doubt that the cephalopods see distinctly for considerable distances, and a careful examination of

the eye of the squids and cuttlefishes has shown them to be remarkably complex and in many respects to be constructed upon much the same plan as those of the vertebrates. As to the other senses not so much is known, but undoubtedly many species of cephalopods are possessed of a shrewdness and cunning not shared by any other invertebrates, save some of the insects and spiders, and are vastly more highly organized than their molluscan relatives.

91. How species originate.-We have now examined a considerable portion of the animal kingdom, tracing its members from their simplest beginnings as single cells, through the formation of colonial types, and up through the sponges, cœlenterates, worms, and mollusks. It is important once more to note that they all perform the functions concerned in nutrition and reproduction, and only these. The differences which exist are those of structure. The Hydra and the clam, for example, perform the same duties, but their bodily apparatus differs widely, and the completeness and perfection of the work varies accordingly. The more the work to be performed by an organism is divided up among especially adapted organs, so that each of the latter has, as far as possible, only one thing to do, the higher is the organism.

As stated earlier in the account, it is believed that the more complex animals arose from the simpler; that if we could trace the history of any of the great groups back toward their first beginnings, we would find them all to have originated from one ancestral form, that in turn owes its descent from yet simpler forms.

Let us see something of how this has come about. We all know that vast numbers of young are born into this world which never come to maturity. It is said that if all the young of the codfish were to live their allotted time, it would be less than fifteen years before the sea would become literally packed with them. Numerous enemies,

the lack of food, and other agencies annihilate the larger part. We also know that no two offspring are exactly alike. They exhibit individual differences. One bird may have a larger bill than another of the same brood which excels in length of wing. As noted above, all the offspring will not attain maturity. Those best adapted to their surroundings will have the best chances of survival. The increased length of bill or wing may be slight, but it may be just this amount which enables the bird to probe deepcr or fly farther and thus secure the requisite amount of food. A premium is placed on length of wing or bill generation after generation, with the result that a long-billed species arises distinct from the long-winged which trace their ancestry back to the same parents. It is the same principle which enables the breeder to increase the swiftness of the race-horse and the strength of the draft-horse, or the gardener to develop from the wild rose the great number of widely different varieties. In the same way other slight peculiarities over very many generations may enable other forms to gradually adapt themselves to still different modes of life. Thus vast numbers of organisms gradually become modified in form and complexity, and are adapted to lives which insure them a comparative degree of safety and less competition with other species.

The above account serves solely for purposes of illustration. No kind of bird has originated in just that way, but as the essential force in all change of form we have the necessity of adaptation of the individual to its surroundings, the death of those who can not be adapted, and the inheritance of the advantage of the parent by its progeny, enabling these in turn to survive and to multiply their own kind.

CHAPTER IX

ARTHROPODS. CLASS CRUSTACEA

92. General characters. In the Arthropods, that is, the crabs, lobsters, shrimps, insects, spiders, and a vast host of related forms, the body is bilaterally symmetrical, and is composed of a number of segments arranged in a series, as in the earthworm and other annelids. A hornlike cuticle, sometimes called the shell, bounds the external surface-in early life thin and delicate, but later relatively thick, and often further strengthened by lime salts. Along the line between the segments this coat of mail remains thin and forms a flexible joint. Appendages also are borne on each segment, not comparatively short and fleshy outgrowths like the lateral appendages of many of the worms, but usually long and jointed (hence the name Arthropod, meaning jointed foot), and variously modified for many different uses.

93. Classification.-The species belonging to this group outnumber the remainder of the animal kingdom. Their haunts also are most diverse. Some are adapted for lives in the sea and fresh water, others for widely different situations on land, and a great number are constructed for a life on the wing. A certain resemblance exists among them all, but the modifications which fit them for their different habitats are also profound, and have resulted in the division of the Arthropods into five classes. The first class (Crustacea) contains the crayfish, crabs, etc.; the second (Onychophora) includes the curious worm-like peripatus (Fig.