48 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
each side of the branchial sac in free communication with 
the external world. For some few sections from No. 170 
forwards this is the state of affairs, and these sections are 
evidently cut through the middle of the stigmata. It is 
clear then that we are dealing with four horizontal rings 
of ciliated cells encircling the middle of the passage. 
Then these passages begin, about section 180, to be 
closed in by the ectoderm growing across the external 
aperture, outside the ciliated cells. It is the most ventral 
extension of the area of large test-producing ectoderm 
cells which effects this change. In fig. 3it is seen that 
the conspicuous ectoderm has exactly reached the lateral 
angles of the triangular body, immediately external to the 
openings (atrial) of the stigmata; in fig. 5 a little thick- 
ened ectoderm is seen on the ventral surface just internal 
to these openings; while in fig. 6, at on the right side, 
the ectoderm is seen to have grown across the opening in 
the form of a fold from each side; and finally at 7, on the 
left side of the same figure, the folds have completely 
united and there is now a wide bridge of tissue, both cov- 
ered and lined by ectoderm, separating the remains of the 
stigma from the exterior. The ciliated cells now die 
away a few sections further forward leaving the lateral 
edges of the branchial sac very much as they were 
immediately posterior to the stigmata (see right side of 
fig. 3 and fig. 7). This series of sections in which the 
stigmata are present shows also the rectum changing 
from a triangular (fig. 3) to a circular lumen (fig. 6), and 
then opening to the exterior at the anal aperture. Fig. 7 
(section 190) is immediately in front of the anus, so the 
rectum is no longer present. 
The ectoderm cells on the lateral walls of the body are 
becoming still larger and more conspicuous (fig. 7), and 
they now extend well on to the ventral surface from each 
