28 THE ORIGIN OF MAN 



canals that run along the center of the arms or 

 radii, these five giving off, in turn, numerous 

 lateral branches which connect with the tube- 

 feet. The sea water has access to this system of 

 canals on the dorsal or upper side of the animal 

 through a large porous, sieve-like plate and the 

 animals can force the water into the tube-feet and 

 so elongate them into action. The Echinoder- 

 mata are divided into two main classes on the 

 basis of their leading a free or sedentary life. 



The great majority of starfishes have a more or 

 less large disk from which radiate at least five 

 arms. They may be long and slender or indis- 

 tinct because of the enlarged interradial areas. 

 It is thought that starfishes first appeared in the 

 Ordovician but it has been found that there are 

 entire specimens in the fossils of Paleozoic rocks. 



Echinids are, as a rule, extremely spiny, many 

 of the spines being movable on ball-and-socket 

 joints. They are generally dome-shaped, the 

 wall of the dome being made up in living forms 



