bottom. It is very cautious and cunning, capturing its prey by stealth, and with weapons which it knows how to conceal. Lying hidden in a bunch of seaweed, in a crevice among the rocks, or in its burrow in the mud, it waits until its victim is within reach of its claws, before striking the fatal blow. The senses of sight and hearing are probably far from acute, but it possesses a keen sense of touch and of smell, and probably also a sense of taste."

Although enclosed in a horny and often very thick and strong armor, the sense of touch is very keen in the Crustacea and in arthropods generally. On many of the more exposed portions delicate hairs or pits connected with the nervous system occur in great abundance. Some of these, usually on the antennæ, undoubtedly serve in detecting odors, but the remainder are considered to be tactile. In the higher Crustacea, such as the crayfish, lobsters, and crabs, ears are usually found, consisting of sacs lined with similar delicate hairs, and containing several minute grains of sand, which in many cases make their way through the small external opening. Vibrations coming through the water gently shake the grains of sand, causing them to strike against the hairs which communicate with the nervous system-a very simple ear, yet sufficient for the needs of the animals.

The eyes of the Crustacea and arthropods in general are either simple or compound. The simple and frequently single eyes usually consist of a relatively few cells embedded in a quantity of pigment and connected with the nervous system. It is doubtful whether they perceive objects as anything more than highly blurred images, and perhaps they merely recognize the difference between light and darkness. The compound eyes, on the other hand, are remarkably complex structures, often borne on the tops of movable stalks, as in the common crabs and crayfishes. Each consists of an external transparent cornea, divided into numerous minute hexagonal areas corresponding to as

many internal rods of cells, provided with an abundant nerve-supply. These latter elements may perhaps represent simple eyes grouped together to form the compound one; and it appears possible that each element may form a complete image of an object, as each of our eyes is known to do. On the other hand, many hold that the complete eye forms only one image, a mosaic, each element contributing its share.

108. Growth and development.-As we have seen, the simpler Crustacea hatch as minute larvæ (Fig. 56), and during their growth to the adult condition are especially subject to the attacks of multitudes of hungry enemies. In the higher forms, such as the crabs, some of these early transformations take place while the young are still within the egg and attached to the parent. Accordingly, the little ones are fairly similar to their parents, and their later history is very well exemplified by the lobster.

The eggs of the lobster are most frequently hatched in the summer months, usually July, after they have been carried by the parent for upward of a year. The young, about a third of an inch in length, at once disperse, undergo four or five moults during the next month, then, ceasing their swimming habits, settle to the bottom among the rocks. At this time, twice their original size, they closely resemble their parents, and their further development is largely an increase in size. "The growth of the lobster, and of every arthropod, apparently takes place, from infancy to old age, by a series of stages characterized by the growth of a new shell under the old, by the shedding of the outgrown old shell, a sudden increase in size, and the gradual hardening of the shell newly formed. Not only is the external skeleton cast off in the moult and the linings of the masticatory stomach, the esophagus, and intestine, but also the internal skeleton, which consists for the most part of a complicated link work of hard tendons to which muscles are attached."

109. Peripatus (class Onychophora).—It is generally believed that the Crustacea, insects, and spiders, together with their numerous relatives, trace their ancestry back to animals that bore a certain resemblance to the segmented worms. Most of these ancient types have

long been extinct, but here and there throughout the earth we occasionally meet with them.

Among the most interesting of these are a few widely distributed species belonging to the genus Peripatus (Fig. 66), but as they are comparatively rare we shall dismiss them with a very brief description. They usually dwell in warm countries, under rocks and decaying wood, emerging at night to feed on insects, which they ensnare in the slime thrown out from the under surface of the head. Their external form, their excretory system, and various other organs are worm-like. On the other hand, the appendages are jointed, and one pair has been modified into jaws. The peculiar breathing organs characteristic of the insects are also present. Peripatus therefore gives us an interesting link between the worms and insects, and also affords an idea of the primitive insects from which the modern forms have descended.

FIG. 66.-Peripatus (Peripatus eiseni). Twice the natural size.

110. The centipeds and millipeds (class Myriapoda). Many of the myriapods-that is, the centipeds and thousand-legged worms --are familiar objects under logs and stones throughout the United States. The first of these (Fig. 67) are active, savage creatures, devouring numbers of small animals, which they sting by means of poison-spines on the tips of the first pair of legs. The bite of the larger tropical

species especially causes painful but not fatal wounds in

man.

On the other hand, the millipeds (Fig. 68) or thousandlegs are cylindrical, slow-going animals, feeding on vegetable

FIG. 67.-Centiped. One-half natural size.

FIG. 68. Thousand-legs or milliped (Julus).
Natural size.

substances without causing any particular damage, except in the case of certain species, which work injury to crops. When disturbed they make little effort to escape, but roll into a coil and emit an offensive-smelling fluid, which renders them unpalatable to their enemies.

All present a great resemblance to the segmented worms, as their popular names often testify; but, on the other hand, many points in their organization indicate a closer relationship to the insects. As in the latter, the head is distinct, and bears a pair of antennæ, the eyes, and two or three pairs of mouth-parts. The trunk is more worm-like, and consists of a number of similar segments, each bearing

one or two pairs of jointed legs. In their internal organization the character of the various systems closely resembles that of the insects, and will be more conveniently described in that connection.

Among the myriapods the females are usually larger than the males. Some of the centipeds deposit a little mass of eggs in cavities in the earth and then abandon them, while others wrap their bodies about them and protect them until the young are hatched. The millipeds lay in the same situations, but usually plaster each egg over with a protective layer of mud. After several weeks the young appear, often like their parents in miniature, but in other species quite unlike, and requiring several molts to complete the resemblance.