The Sea- Urchin. 39 



What plates did we find on the star-fish ? 



Plates that made a network on the back. Two rows 

 of perforated plates on each arm. One row of unper- 

 forated plates on each side of the perforated ones. 



What is the use of the holes between the perforated 

 plates of the star-fish ? 



The tube-feet pass through them. 



How are the rows of plates arranged in the sea-urchin ? 

 First there are two rows of large plates, then two rows 

 of small plates. 



How many rows in all ? 

 Ten rows of each kind. 



Where are the eye-openings ? 



At the end of the rows of small plates. At the end of 

 the perforated plates. At the end of the plates that the 

 tube-feet come out through. 



Where are the eyes of the star-fish ? 

 At the point of each ray. 



Are they at the end of any rows of plates ? 

 Yes, at the end of the rows of perforated plates. 



And where are they in the sea-urchin ? 



At the end of the rows of perforated plates, too. 



Where are the egg-openings ? 



At the end of the rows of large plates. 



But we have not found any nerves, some child suggests. 

 No ; for the sea-urchin has them safely tucked away on 

 the inside of its shell. 



