more or less nearly completely to form a single ganglion, and the nerve-cords are partially fused, or at least lie close together. In addition there is a smaller sympathetic system composed of a few small ganglia and certain nerves running from them to the viscera, this system being connected with the main or central nervous system. In this group the organs of special sense reach for the first time a high stage of development. Compound eyes are peculiar to Arthropoda. The heart lies above the alimentary canal. Respiration is carried on by gills in the aquatic forms, and by a remarkable system of air-tubes or tracheæ in the land forms (insects). The sexes are usually distinct, and reproduction is almost universally sexual. Most of the species lay eggs. The Arthropods are animals of a high degree of organization. The extremely diverse life-habits of the various kinds among them have led to much modification and to great specialization of structure. The course of development, too, is made very complicated by the elaborate metamorphosis undergone by many of the members of the branch. We shall study the Arthropoda by getting acquainted with a few examples of each class and thus learning the special class characteristics. CLASS CRUSTACEA: CRAYFISHES, CRABS, LOBSTERS, ETC. THE CRAYFISH (_Cambarus_ sp.) =Structure.=--The structure of the crayfish has been already studied (see Chapter IV and figs. 3 and 4). =Life-history and habits.=--Crayfish frequent fresh-water lakes, rivers, and springs in most parts of the United States. Many of them perish whenever the small prairie ponds dry up. But some burrow into the earth when the dry season comes. There may be noticed in meadows where water stands for certain seasons of the year many scattered holes with slight elevations of mud about them. These are mostly the burrows of crayfish. During the dry season the crayfish digs down until it reaches water, or at least a damp place, where it rests until wet weather brings it to the surface once more. One of these burrows, followed in digging a mining shaft, extended vertically down to a distance of twenty-six feet, where the crayfish was found tucked snugly away.