Echinodermata. 



339 



name of sea eggSo In a cleaned shell, or corona, as it is 

 sometimes called, there are found ten double rows of cal- 

 careous plates, making twenty in all. Every alternate 

 double row is closely perforated for the passage of the 

 tube feet. These perforated plates are called the ambu- 



FIG. 191. SKA URCHIN. 



The heavy projections are the spines; the long, slender ones are the tube feet. 



lacral plates. Both the ambulacral and the interambula- 

 cral plates bear rounded elevations, on which the spines 

 were borne. Each spine has a hollow in its base, which 

 fits over an elevation on the shell, making a ball-and-socket 



