Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/50

38 composed of a head and a body; sometimes the head and body coalesce. The head is furnished with fine hairs arranged in different manners, and when these cilia are in action they look like wheels. The other end of the body terminates in a jointed foot. Both the wheel-organs and foot can be drawn within the case in which the body of the Rotifer is inclosed. This case resembles that of the Crabs. The Rotatoria possess the water-vascular system of the Worms, as described in Aspidogaster. The group is intermediate in its structure between the Soft Worms, the Annelida, and the Crabs,—the Rotatoria having been considered to belong to each of these groups by different naturalists. They represent very naturally that point of the tree where the Soft Worms end and the Crabs begin. Before leaving the Articulated Worms, the position of the Artisca must be noticed. They have been called Tardigrada, from the slowness of their movement; they are usually considered as nearly related to the Spiders; others have looked upon them as Annelids, while some have considered them as the links between the Soft Worms and Rotatoria. They are placed, therefore, near these groups, without assigning to them a definite position. From the difificulty experienced in their classification, the Rotatoria and Artisca afford a striking proof of the truth of an evolution of these worms in some such manner. Having called attention briefly to the Soft and Articulated Worms, we pass to the last division, the Sac-worms, which includes the Bryozoa and Tunicata.