Plate X. 



Figs. 1 — g. Successive vertical sections, parallel to the sagittal plane, through an embryo in the stage III, 

 prepared out of the egg-membrane. *°/i. 



Fig. I. The section goes through the outer part of the tentacle sheath. 



Fig. 2. Further inwards, the section going through the coiled-up tentacle which lies within 

 the tentacle .sheath. (Comp. PI. II. Fig.8, PI. VIII. Fig. 12). The outer end of the entodermal 

 sacs is seen in the section; here the large entodermal cells are lying in two strands, as 

 in the branching peripheral canal system. 



Figs. 3 — 7 give successive sections through the tentacle apparatus and the entodermal sacs. 

 Fig. 8 is inside the tentacle basis. In fig. 6 is seen the direct connection between the 

 lumen of the entoderm sacs and the diverticula in the tentacle basis. Figs. 7—8 show 

 the continuation of the costal sacs as low furrows running towards the apical organ. 



Fig. 9. Median section, through the apical organ, the excretory canals, infundibulum and oeso- 

 phagus. (Comp. PI. VII. Fig. 10). The flat epithelial ring above the apical organ is the 

 epidermis arching over the apical organ — not quite normally, due to the contraction on 

 preservation. The situation of the costse within the body in figs. 7— 9 is only apparent; 

 a comparison with the horizontal sections in PI. IX easily explains how this arrangement 

 has arisen through the intercostal parts arching over the deep lying costse. 



— 10. The apical organ of an embryo in stage III; seen in side view, from the sagittal side. 



Above the otolith is seen the polar field. "°!t,. 



— II. Tlie same as fig. 10 seen from above; showing one excretory pore, the other being indis- 



tinct in the preparation. ^20^^. 



