MUSEUM OF COMPARATIVE ZOOLOGY. 259 



is rifht in considering the arm-bones or disks as parts whose homology is 

 obscure ; certainly they are not properly ambulacral plates, as Job. Midler 

 thought, because the tentacles, with their water-system, lie above the ambu- 

 lacral plates in starfishes, whereas in Ophiurans they lie below the arm-bones 

 and above the under arm-plates, which latter are plainly the true ambulacral 

 plates, not only from their position, but from their early formation, which 

 corresponds to that of the same parts in the starfishes. 



