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for lining their burrows, or for the construction of webs of the most diverse patterns.

132. The habits of spiders.-Many species of spiders, some of which are familiar objects in fields and houses, construct sheets of cobweb with a tube at one side in which they may

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FIG. 83.-A tarantula-spider (Eurypelma lentzii). Natural size. Photograph by A. L. MELANDER and C. T. BRUES.

lie in wait for their prey or through which they may escape in times of danger. In the webs of the common orb- or wheel-weavers several radial lines are first constructed, and upon these the female spider spins a spiral web. Resting in the center of this or at the margin, with her foot on some of the radial threads, she is able to detect the slightest tremor and at once to rush upon the entangled captive.

Some of the bird-spiders and their allies, living in tropical America, and attaining a length of two inches, construct web-lined burrows in the ground. From these they stalk their prey, which consists of various insects and even

small birds. These are almost instantly killed by the poisonfangs, and are then carried to the burrow, where the juices of the body are extracted.

The trap-door spiders of the southwestern section of the United States also dig tunnels, which they cover with a closely fitting lid composed of earth. Raising this they come out in search of insects, but if sought in turn, they dash into the burrow, closing the door after them, and holding it with such firmness that it is rarely forced open. If this should happen, there are sometimes blind passage-ways, also closed with trap-doors, which usually baffle the pursuer.

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FIG. 84.-Trap-door spider and burrow (Cteniza).

Finally, there are among the thousand species of spiders in the United States a considerable proportion which construct no definite web. Many of these may be seen darting about in the sunshine on old logs and fences, often trailing after them a thread which may support them if they fall in their active leaping after insects.

133. Breeding habits.-The male spiders are usually much smaller than the females, and some species are only onefifteenth as long as the female and one one-hundredth of its weight. They are usually more brilliantly colored, more active in their movements, yet rarely spinning their own. webs and capturing their own food, preferring to live at the expense of the female. At the breeding season the males of several species make a most interesting display

of their colors, activity, and gracefulness before the females; and the latter, after watching these exhibitions, are said to select the one who has "shown off" in the most pleasing fashion. The life after this may be stormy, resulting in the death of the male; but ordinarily the results are not so disastrous, and in a little while the female deposits her eggs in cases which she spins. In these the young develop, sometimes wintering here, and emerging in the spring to scamper about in search of food, or to drift through the air to more favorable spots on fluffy masses of cobweb.

Few groups of animals are more interesting objects of study and more accessible. Their bites are rarely more serious than those of the mosquito-never fatal; and a

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careful study of any species, however
common, will undoubtedly bring to
light many interesting and unknown
facts.

134. The mites and ticks.-The
mites and ticks are the simplest and
among the smallest of the animals
belonging to this group. To the at-
tentive observer they are rather com-
mon objects, with homes in very dif
ferent situations. Some occur on liv-

FIG. 85.-The itch-mite (Sar- ing and decaying vegetation, in old flour and unrefined sugar, while others live in fresh water and a few in the sea. Almost all tend toward parasitism. Some of the insects which they pierce and destroy are a pest to man, but on the other hand some are intolerable owing to the diseases they produce.

As to other parasitic organisms, degradation of structure is manifest. The respiratory system, so important to the active life of the insects, may be absent, the animal breathing through its skin. The circulatory system may be wanting, the blood occupying spaces among the various organs being swept about by the animal's movements. And many

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other peculiarities have arisen which fit them for their different modes of life.

135. The king crab (Limulus).-The king crab may be found crawling over the bottom or plowing its way through the sand and mud in many of the quiet bays from Maine. to Florida. The large head and thorax of these animals are united into a horse

shoe-shaped piece, behind which lies the triangular abdomen. On the curved front surface of the former are a pair of small median eyes, and farther outward are two larger compound ones. On the ventral side are six pairs of appendages, instrumental in capturing and tearing the small animals that serve as food, and functioning in connection with the terminal spine as locomotor organs. On the ventral surface of the abdomen are numerous plate-like flaps which serve in respiration, and in the imperfect swimming movements in which these animals occasionally indulge.

FIG. 86. The king or horseshoe crab (Limulus polyphemus).

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These relatively large and clumsy creatures are the remnant of a great number of strange, uncouth animals that inhabited the earth in past ages. Many of them show a close resemblance to the scorpions. The anatomy and development also show certain points of resemblance, and by some are thought to give us an idea of the ancient type of spiderlike animal from which the modern forms have descended.

CHAPTER XII

ECHINODERMS

136. General characters.-The division of the echinoderms includes the starfishes, sea-urchins, serpent- or brittlestars, sea-cucumbers, and crinoids or sea-lilies. All are marine forms, and constitute a conspicuous portion of the animals along almost any coast the world over. From these shallow-water situations they extend to the greatest depths of the ocean, and the bodily form possesses a great number of variations, adapting them to lives under such diverse conditions; and yet there is perhaps no group of organisms so clearly defined or exhibiting so close a resemblance throughout. At one time it was thought that their radial symmetry was an indication of a close relationship to the cœlenterates, but more careful study has shown them to be much more highly developed than this latter group, and widely separated from it. A skeleton is almost always present, consisting of a number of calcareous plates embedded in the body-wall, and often supporting numbers of protective spines, which fact has given to the group the name Echinoderm, meaning hedgehog skin.

137. External features.-The body of a starfish (Fig. 87) consists of a more or less clearly defined disk, from which the arms, usually five in number, radiate like the spokes of a wheel. At the center of the under side the mouth is located, and from it a deep groove, filled with a mass of tubular feet, extends to the tip of each arm. Innumerable calcareous plates firmly embedded in the body-wall serve

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