STUDIES IN THE EMBRYOLOGY OF EOHINODERMS. 419 
remains open to the anterior enterocoel, from which it probably 
arises. 
Crinoids. — Only one anterior enterocoel is present. The 
hydrocoel is at first connected with this anterior enterocoel, but 
subsequently becomes independent. The water-pore opens to 
the anterior enterocoel, and a pair of posterior enterocoels lie 
beside the stomach. 
Holothurians. — The left anterior enterocoel appears to be 
present, but rudimentary, and connected from the first with 
the hydrocoel. Two posterior enterocoels exist as in other 
groups. 
From these facts we may arrive at the following conclu- 
sions : 
(1) A pair of anterior enterocoels was probably originally 
present in all Echinoderms. So long as the left anterior 
enterocoel of Ophiurids and Echinids was confused with the 
hydrocoel, doubts were frequently expressed as to whether it 
ever had a fellow on the right side ( 4 , p. 458; 11 , p. 609; 
17 , p. 141 ; 26 , p. 49) ; and even when this was admitted to 
exist, it was hinted that this bilaterally symmetrical arrange- 
ment might be pathological ( 17 , p. 142). However plausible 
this supposition may formerly have seemed, it appears to me 
absolutely untenable in the face of the new evidence here 
advanced. Pathology may account for such obviously mon- 
strous forms as that observed by Metschnikoff ( 19 , p. 64), but 
the term is clearly inapplicable to a condition which obtains 
with the utmost regularity in every individual member of two 
groups (Ophiurids and Echinids). Nor is it any more satis 
factory to assume that a portion of the enterocoel is cut off on 
the right side merely for the sake of symmetry ( 26 , p. 50) and 
is then allowed to atrophy without further development. The 
only tenable view, as it seems to me, involving a secondary 
origin for these anterior enterocoels, is that they are derived by 
a species of segmentation from such a condition as is found in 
many Asterids, in which the oesophageal and gastral sections 
of the coelom are continuous on each side of the body ; but the 
apparently primitive character of the Bipinnaria, in which the 
