NOTES ON THE STliUCTURE OF OIKOPLEURA. 39 



each side of the branchial sac in free communication with 

 the external world. For some few sections from No. 170 

 forwards this is tlie 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. 3 it 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 A 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 i, 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 



