152 
Fig. 
“ 
“e 
tii 
10. 
11. 
12. 
13. 
14. 
15. 
BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. 
Lateral view of a sea-urchin of about the same age, showing alternating 
ambulacral tubes and spines. 
A young sea-urchin, under slight pressure, showing a central and five 
peripheral plates in its apical region. 
A young sea-urchin before the absorption of the pluteus, showing two 
ambulacral zones. Oral view. 
Three contiguous plates from a very young sea-urchin, showing charac- 
teristic double calcification. 
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. 
Aboral view of a young Echinarachnius obtained by dredging. 
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. 
