76— HENRY BURY. 
a group of cells situated over the middle line of the esophagus, 
to the right and rather in front of the water-pore. A schizoccel 
space soon appears in this mass, and rapidly increases in size 
(fig. 29). It is certainly distinct from the ccelom in its origin, 
and, to the best of my belief, throughout larval and adult life. 
The similarity of its origin and position, as well as its relation 
to the dorsal organ (to be described later) lead me to regard it 
as homologous with the “ dorsal sac” of Asterids, and by this 
name I shall in future refer to it. 
In late Plutei its floor projects far into its cavity (fig. 30), 
but the extent to which this is seen in sections varies consider- 
ably in different specimens, apparently depending on the 
methods used in preservation. As far as I can judge from a 
careful study of the living animal under high powers of the 
microscope, it is this part, and not the cavity as a whole, which 
pulsates. This pulsation, I may mention, is certainly continued 
in the earliest post-larval stages, though whether it occurs in 
the adult I am unable to say. 
The left anterior enterocel forms at a fairly early stage a 
large ampulla where the pore-canal and water-tube open into 
it (fig. 31). In later stages it extends backwards for some 
distance alongside the water-tube—further than in fig. 30, but 
exactly how far I am still doubtful. 
The dorsal organ is rather difficult to trace with accuracy, 
but subsequent stages make it almost certain that a mass of 
cells (usually filled with yellow granules), projecting into the 
dorsal sac on the one hand (fig. 80) and into the ampulla on 
the other (fig. 31), is the rudiment of this organ. Besides 
this, a cord of cells is frequently noticeable (fig. 80) lying 
alongside the water-tube (on the ventral side of this structure 
in the larva), which may be a part of the dorsal organ; but I 
am unable to distinguish it clearly in transverse section, or to 
trace it into connection with the above-mentioned mass of cells 
with yellow granules. The aspect of all these parts varies 
much with the methods employed, and great care should be 
taken to kill the animal in an extended condition. Where 
this is not done the great mass of transparent tissue projecting 
