152 BULLETIN OF THE MUSEUM OF COMPAEATIVE ZOOLOGY. 



Fig. 10. Lateral view of a sea-urchin of about the same age, showing alternating 

 ambulacra! tubes and spines. 



11. A young sea-urchin, under slight pressure, showing a central and five 

 peripheral plates in its apical region. 



12. A young sea-urchin before the absorption of the pluteus, showing two 

 ambulacral zones. Oral view. 



13. Three contiguous plates from a very young sea-urchin, showing charac- 

 teristic double calcification. 



14. A young Echinarachnius raised from a pluteus. The sea-urchin has 

 absorbed all the soft parts of the pluteus. Three limestone rods, spi, 

 still remain unabsorbed. Lateral view. Oral region below. 



15. Aboral view of a young Echinarachnius obtained by dredging. 



16. Oral view of an older stage also obtained by dredging. The animal from 

 which Figs. 15 and 16 were drawn do not differ much in size. 



