[8] The author recognizes the untenability of the group Vermes as a group co-ordinate with the other branches of the animal kingdom, and that "Vermes" has been discarded in modern text-books. But because of the very scant consideration which can be given the various kinds of worm-like animals the course of the older text-books will be followed, and all of the worm-like animals, as far as referred to in this book, be considered under the group name Vermes. CHAPTER XX BRANCH ARTHROPODA: CRUSTACEANS, CENTIPEDS, INSECTS, AND SPIDERS The great branch Arthropoda includes a host of familiar animals. It contains more species than any other branch of the animal kingdom. To it belong the crayfishes, shrimps, crabs, lobsters, water-fleas, and other animals which compose the class Crustacea; the centipeds and thousand-legged worms which compose the class Myriapoda; the true or six-footed insects forming the class Insecta, which includes nearly two-thirds of all the known species of animals; and the scorpions, mites, ticks, and spiders which constitute the class Arachnida. There is also a fifth class in the branch Arthropoda which includes a few species of animals unfamiliar to us but of great interest to zoologists. All these varied kinds of animals have a body on the annulate or segmented type-plan, like that shown by most worms, but they differ from the worms in possessing jointed appendages, used for locomotion or food taking. There is typically or racially one pair of these jointed or segmented appendages on each segment of the body, but in all of the Arthropoda some of the segments have lost their appendages. The body is covered by a firm cuticle or outer body-wall called the exoskeleton. This exoskeleton serves not only to enclose and protect the soft parts of the body but also for the attachment of the body muscles. It may be flexible as in the sutures between the body-segments in most insects, or hard and rigid as in the sclerites of the segments. The firmness is due primarily, and in the insects usually solely, to a deposit in the cuticle of _chitin_, a substance probably secreted by the underlying cells of the true skin, or it may be due chiefly, as in the crabs, to a calcareous deposit. In such cases it becomes a veritable armor. The internal organs of the Arthropods show a more or less obvious segmentation corresponding with the segmentation of the body-wall. The alimentary canal runs longitudinally through the center of the body from mouth to anal opening. The nervous system consists of a brain lying above the œsophagus and a double nerve-chain running backward from beneath the œsophagus, along the median line of the ventral wall, to the posterior extremity of the body. This ventral nerve-chain consists of a pair of longitudinal commissures or cords and a series of pairs of ganglia, arranged segmentally. The two ganglia of each pair are fused