41 6 MORGAN. [Vol. V. 



thick, and also thickened between x and y and x and z. The 

 thickened ectoderm of the band is richly ciliated, and the region 

 just below the mouth has a band of long cilia. We may say in 

 general that the area within the circumoral (longitudinal) band 

 has very thin walls, while the extra-oral ectoderm is relatively 

 thicker. Reference to Fig. 6 will make clear these areas of 

 Fig. 33. The oesophagus (ce) is quadrilateral in outline ; and the 

 cells, which are somewhat crowded, have long cilia which almost 

 fill up the lumen. On its outer walls are mynerous mesenchyme 

 cells. The anterior enteroccel is cut transversely above the 

 upper end of the proboscis vesicle, and below the thickened 

 ring. 



Fig. 34 is from a section about the middle of the embryo, as 

 shown by line 34, Fig. 6, PI. XXIV. The large stomach is seen 

 to fill up almost the entire interior of the section. In the dorsal 

 and ventral line (above and below in the figure) the ectoderm is 

 thick, and belongs to the extra-oral area. Four ciliated bands 

 bound the edges of this ectoderm. The ectoblast along the 

 sides is extremely thin, and belongs to the circumoral area. In 

 the mid-dorsal line the ectoblast has a well-developed nerve-fibre 

 layer on its inner surface, and this is the area from which in 

 later stages the nerve cord is formed. Lastly, Fig. 35 passes 

 between the longitudinal and circular bands, as shown in line 

 35, Fig. 6, PI. I. The section passes through the posterior part 

 of the stomach, near to its connection with the "intestine." 

 The posterior pairs of body cavities are seen at the sides, almost 

 encircling the digestive tract. Each contains a narrow lumen, 

 and the walls are formed of a single layer of columnar cells. 

 The ectoderm in this section (which is extra-oral) is exceedingly 

 thick. At one point, owing to a slight obliquity of the section, 

 a portion of the circumoral area is cut. 



In Fig. 36 is shown a part of a section of Tornaria at right 

 angles to the last series, passing through the body from side to 

 side, and is drawn to a much larger scale than the last. The 

 ectoderm is better shown than in the preceding figures. In the 

 upper right-hand portion of the figure the ectoderm is exceed- 

 ingly thin, formed of flattened ectoderm cells. At / is cut the 

 lower limb of the circumoral band, and then comes the thick- 

 ened extra-oral ectoderm, lying between the longitudinal and 

 circular bands, and corresponds to that shown in Fig. 35. It 



