ON WANDERING CELLS IN ECHINODERMS. 115 
“canal aquifére annexe,” or axial sinus of Kchinids, is similar 
in its development. 
In Ophiurids an axial (perihemal) sinus exists ; but accord- 
ing to Hamann (1, ‘ Die Ophiuren”) it does not communicate 
with the water vascular apparatus in the adult. 
This view seems to me to reconcile the discrepancies in the 
descriptions which have been published of the anatomy of 
the region ; the differences having apparently arisen from the 
retention or loss of the embryonic condition of the individual 
examined. 
Lastly, we have to deal with Sarasin’s (19) conception that 
the organ is to be regarded as a nephridium, with nephrostomes 
connecting its cavity with the general body-cavity. Taking 
into consideration the arrangement found in Asterids and 
Ophiurids, and believing that the organ is homologous through 
these groups, it is more probable that the communications are 
secondarily acquired, and have nothing to do with nephro- 
stomes; moreover, if study of development shows that the 
cavity of the organ and Sarasin’s “ureter” are in reality 
derived from enterocele cavities, the dorsal organ will no more 
fit in with what is understood as a true ‘‘nephridium” than 
will Hartog’s (17) view that the whole water vascular system 
is in reality a left nephridium, a view which Cuénot has very 
properly criticised (5). 
I have searched for “ nephrostomes ” in one large specimen 
of Spatangus purpureus, and have been unable to find a 
trace of any such structures. Three specimens! of Echinus 
sphera have been examined with a similar object. There 
are numerous pits on the surface ; these are lined by more or 
less cubical epithelium. I have failed to find any communica- 
tion between the cavity in the organ and the lumina of these 
pits by careful examination of both longitudinal and transverse 
serial sections. The pits are cecal, and their function is, 
perhaps, to allow wander-cells to pass from the reticulum of 
the organ to the celom, and vice versa, without traversing 
’ One preserved with sublimate, one with Flemming’s mixture, and the 
third with osmic acid. 
