ECHINODERMATA. 



207 



animal into halves? Where are the ends of these lines 

 (axes) ? The arm is known as a radius; the space between 

 two arms is an inter-radius. 



FIG. 62. Starfish, after Leuckart and Nitsche. 



Questions on the Figure. How would you describe the symmetry 

 of this form? Compare the figure with the diagrams and the text, 

 and identify all the structures and regions which show in the figure. 

 Compare with figures of other species of starfish and note the contrast 

 in disc, arms, etc. 



The External Organs. Which is the upper surface in 

 life? Reasons for your view? Locate the mouth. This 

 surface is known as the oral surface; the opposite is the 

 aboral. Examine these two surfaces with care and note 

 every structure found on either surface, and the differences 



