STUDIES IN THE EMBRYOLOGY OF ECHINODERMS. 425 
enterocoel, which I have marked “ ampulla,” and this in its turn 
opens to the exterior by a conspicuous median pore. Also in 
the median line, and partly hidden by the pore, is a large pul- 
sating vesicle, which is doubtless the same as that seen by 
Metschnikoff ; it is overlaid by a reticulated calcareous plate 
(not represented), which surrounds the pore, and makes it 
extremely difficult to determine the relations of the subjacent 
parts. That the pore does not open directly into the pul- 
sating vesicle I am almost certain, while I have seen its open- 
ing into the non-contractile ampulla in a large number of 
specimens. It is certainly, however, possible that there may 
exist a communication between the ampulla and the pulsating 
vesicle which has escaped my notice ; the wall separating them 
is undoubtedly very thin, and a small valvular aperture in it 
would be exceedingly hard to see in the living animal, while 
it would be practically impossible to detect in sections. I 
have not as yet been successful in tracing the origin of this 
pulsating vesicle, but, as far as I can make out, it is at first 
more widely separated from the anterior enterocoel than in the 
stage figured, and I have no reason to think that it is derived 
from this enterocoel ; I am more disposed to believe that it is of 
schizocoel origin, and that it is at no time connected either 
with the water-pore or the ampulla. 
Most of the parts represented in fig. 9 are subject to con- 
siderable variation in size, not only in different forms of 
Plutei, but even in Plutei belonging to the same species ; it 
may, however, be stated generally that the right anterior 
enterocoel and the anterior continuation of the left anterior 
enterocoel (i. e. all except the ampulla) are difficult to see in the 
living animal, since they lie rather under the oesophagus, 
though they are easily visible in sections. In the specimen 
figured the water-tube (stone-canal) and the thick-walled 
tube from the water-pore (pore-canal) are continuous on one 
side ; but in some other Plutei, and especially in Spatangids, 
they are more or less widely separated ; in fact Pewkes (9) has 
evidently seen only the water-tube, and has described its 
opening into the anterior enterocoel as the water-pore; the 
