Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/54

40 rest of the animal kingdom. By looking at Tree IV. may be seen the stem of the Gephyrea, giving origin to the Star-fishes, the simplest of the Echinodermata. In the Annelida will be found the roots of the Tracheate, or tube-breathing Articulata, while the Rotatoria lead equally naturally to the Crabs. The Articulated Worms furnish us with the roots of the Echinodermata and Articulata, while the Sac-worms contain the foreshadowing of the Vertebrata and Mollusca.



This division of the animal kingdom includes the Starfishes, Feather-stars, Sea-urchins, and Sea-cucumbers. (Figs. 41, 43, 44, 45.) Every one who has visited the seashore is familiar with the appearance of the star-fish. (Fig. 41.) From the mouth, which lies in the centre of the body, fork out five arms, which run insensibly into each other, the mouth lying in the middle of the space formed by the union of the diverging or radiating arms. The number of arms in some star-fishes is as many even as forty, but the most common number in all the Echinodermata is five. Each arm in the star-fish is composed of movable segments. There exists also a water-vascular system, which terminates externally in suckers, serving as organs of locomotion. There is a rudimentary blood-vessel system, beginning as a tube in the body of the star-fish, and which courses outwards. On the under surface of the arm is found a fine nervous thread, coming from a ring surrounding the mouth, and, finally, at the free end of each arm eyes are found. The arm of a star-fish is, in fact, a worm; not simply resembling one, but structurally the same, the segmentation, the water-vascular system, the nervous cord in each arm of the star-fish being exactly the same as that of an articulated worm. The star-fish has probably been produced through