THE STARFISH AND SOME ALLIES 



235 



drical, but without calcareous plates that touch one another 

 and keep the form constant. The only trace of a skeleton 

 comparable to that of the sea urchin consists of scattered 

 bits of calcareous secretions of definite form beneath the 



Fig. 127. Photograph of a living sea cucumber. (Natural size) 



skin. Although the sea cucumber is radially symmetrical, 

 having the five double rows of tube feet along the cylinder, 

 we can speak of the anterior and posterior ends because the 

 animal lies flat. 



The anterior region is the oral region. At the center we 

 observe the round mouth, about which are the ten branch- 



