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

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

FIG. 88.—The itch-mite (Sar- ing and decaying vegetation, in old flour and unrefined sugar, while oth

coptes scabei).

ers 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

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

141. 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. 89.-The king or horseshoe crab (Limulus polyphemus).

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

142. 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.

143. External features.-The body of a starfish (Fig. 90) 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

for the protection of the internal organs, and at the same time admit of considerable movement.

In the brittle-stars (Fig. 91) the central disk is much more sharply defined than in the preceding forms, and the long snake-like arms are capable of a very great freedom of movement, enabling the animal to glide over the sea-bottom, or through the crevices of the rocks, at a surprising rate. In several species, otherwise closely resembling those

in Fig. 91, the arms divide repeatedly. These are the socalled basket-stars, living in the deeper waters of the sea, where they, like other brittle-stars, act as scavengers and devour large quantities of decomposing plant or animal remains.

At first sight the globular spiny sea-urchins (Fig. 93) would scarcely be recognized as close relatives of the starfishes. A closer examination, however, shows the mouth to be located on the under side of the body; from it five rows of feet radiate and terminate close to the center of the dorsal side, and the arrangement of the plates forming the

skeleton indicate that the sea-urchin is comparable to a starfish, with its dorsal surface reduced to insignificant proportions.

In the sea-urchins the calcareous plates possess a great regularity, and are so closely interlocked that they prevent

any motion of the body-wall. Also, each plate is usually provided with highly developed spines, movable upon a balland-socket joint. These spines serve for locomotion, and, in some instances, for conveying food to the mouth. A considerable number of sea-urchins show an irregularity in form which destroys to a corresponding degree the radial symmetry. This is due to various causes, but especially to a compression of the body, which, in the "sand-dollars,"