66 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
anatomy and histology between the Synaptidae and other holothu- 
rians seem to indicate that both had a common ancestry. 
The water-vascular system both in the Molpadiidae and Synaptidae 
has undergone a marked degeneration in adaptation to a life of 
burrowing in the sand; in the former the radial canals and a few 
rudimentary ambulacra only remain; in the latter even the radial 
canals, as Ludwig has shown, are lost during the development of 
the individual. 
