NOTES ON THE STEUCTURE OF OIKOPLEURA. 37 



a few sections further forward, thus bringing us to the 

 anterior end of the very extensive hermaphrodite repro- 

 ductive system (compare PL IV. fig. 9). 



The gradual increase of the dorsal area of large ectoderm 

 cells, and of the thickened test covering it, is very notice- 

 able, and the point where ectoderm and test join and thin 

 off into a delicate membrane ( X in PI. II. fig. 5) can be 

 seen to advance further and further down the wall of the 

 body until in section 155 (PI. III. fig. 1) it has almost 

 reached the ventral surface. We are now approaching the 

 anterior end of the stomach. This extensive cavity has 

 been gradually changing its shape in the sections from a 

 vertical bilobed to a single horizontal space lying across 

 the middle of the body (see PL III. fig. 1) between the 

 dorsal branchial sac and the ventral rectum. In the next 

 few sections the walls are found encroaching irregularly 

 upon the lumen, and in section 163 (PL III. fig. 2) the 

 lumen has completely disappeared, and a couple of sections 

 further forward (PL III. fig. 3) no trace of the wall of the 

 stomach is left. 



Just before we finally leave the stomach behind we come 

 upon the posterior end (really the morphological anterior 

 part) of the great ventral appendage or "tail" (PL III. 

 fig. 1, app.). Consequently the tail joins the body just 

 about on a level with the anterior end of the stomach. 

 No trace of the tail is visible in section 125 (PL II. fig. 5,) 

 while it is seen just completely separated off from the 

 ventral body-wall in section 155 (PL III. fig. 1). It first 

 appears a few sections behind this, and it is the lateral 

 edges of the tail which first become free, and lastly the 

 median part lying between the notochord and the rectum 

 separates off from the ventral wall of the body. From this 

 point the tail is present in all the sections forward to the 

 front or oral end of the body, and even for a considerable 



