40 EVOLUTION OF LIFE. 
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 nat- 
urally to the Crabs. The Articulated Worms furnish us 
with the roots of the Echinodermata and Articulata, while 
the Sac-worms contain the foréshadowing of the Verte- 
brata and Mollusca. - 
ECHINODERMATA. 
This division of the animal kingdom includes the Star- 
fishes, Feather-stars, Sea-urchins, and Sea-cucumbers. 
(Figs. 41, 43, 44, 45.) Every one who has visited the sea- 
shore 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. 
TI 
ere is a rudimentary blood-vessel system, beginning as a 
tube in the body of the star-fish, and which courses out- 
wards. 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 resem- 
bling 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 
