The Sea - Urchin. 37 



shoald always be the motto in Boienoe lessona, any attempt at haste 

 will be especially fatal here. 



Taking the bare shells we first see that they are com- 

 posed of many parts called plates. As we hold them up 

 to the light and look inside, we discover a great many 

 small openings in them, like fine pin-holes. There are 

 ten rows of these openings, or five double rows, and sharp 

 eyes will see that there are also five double rows of small 

 plates through which these holes pass. These are the 

 perforated plates, and the holes are for the tube-feet, as 

 in the star-fish. On each side of a double row of small 

 plates is a double row of large plates without openings. 

 These we call the unperforated plates, and we find five 

 double rows of these also. We remember that there are 

 ten rows of perforated plates in the star-fish, also, as well 

 as ten rows of unperforated plates. 



We now find an opening in the sieve, and we observe 

 that the sieve is at the end of two rows of large plates. 

 From the inside of the shell four more openings can be 

 seen at the end of the other double rows of large plates. 

 These are the openings through which the eggs pass out 

 into the water. The egg-openings are in little plates 

 shaped somewhat like the sieve, and really five in number 

 since the sieve is on one of them. 



Five more tiny holes alternate with the egg-openings, 

 and stand at the end of every two rows of perforated 

 plates. These are eye-openings, and are in very small 

 plates called eye-plates. The young sea-urchin has five 

 eyes in these places, but when fully grown he has only 

 these orifices, through each of which a tube-foot passes 

 out. Inside the circle of eye-plates and egg-plates is a 

 little disk of tough skin containing minute plates. 



