Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/60

44 from the Soft Worms, with which they are most closely allied, is in harmony with the views of the most eminent naturalists of the present day.

We turn now to a consideration of the Articulata, so called from their bodies being composed of distinct pieces jointed or articulated together. They include the Centipedes, known as the Myriapoda, from their numerous feet, the Spiders, or Arachnida, with eight feet, and the Insects, which have only six feet. We will pass over for the present the Crustacea, or last order of Articulata, and confine ourselves for the moment to the Centipedes, Insects, and Spiders (Figs. 46, 48, 49), which agree in breathing by means of fine tubes opening externally. These tubes pass from the walls of the body, getting smaller and smaller, until they are lost in a net-work. In the inside of these tubes, arranged in the form of a spiral, is found a delicate wire, which serves to keep the tubes expanded. This respiratory system is known as the Tracheate: hence the Centipedes, Insects, and Spiders are joined in one division, the Tracheata. A Myriapod, or Centipede (Fig. 46), is composed of numerous segments resembling a Nereid; in fact, it has been observed that the Myriapoda are to the land what the Annelida are to the water. In the Insect (Fig. 48) we can distinguish only three segments, known as head, thorax, and abdomen. There are seen usually in Insects two pairs of wings, less often one pair, and in some cases none are apparent. In the Spider (Fig. 49) the head and breast are soldered into one piece, known as cephalo-thorax. So in the Arachnida we find only two segments. While the Myriapoda, Insecta, and Arachnida breathe by tracheae, the Crustacea, including the Crabs, Lobsters, etc., breathe by gills. They live in the water, and are of every size,