Plate Vll. 



Fig. I. Vertical section, parallel to the sagittal plane, through the apical organ of a grown specimen 



(Coinp. PI. V. Fig. i6); showing the two excretory canals. The right one was not found 

 open in its whole length, undoubtedly on account of a slight curve in its course (due to 

 the preservation?). The apical organ is seen to be quite rudimentary; no distinct otolith 

 is seen (Comp. F'ig. 2), but a pair of otolith cells are seen within the cell mass. — The 

 three larger nuclei seen at the upper side of the organ to the left side probably represent 

 the zone of cells forming the cupule, which has disappeared. Cilia were not to be seen 

 here in the excretory canals. (Not quite reliable in all the histological details, the preser- 

 vation being insufficient for that purpose). ''5/i- 



— 2. Vertical section, in the transversal plane, through the apical organ and the oesophagus 



of a grown specimen. TJie right side of the upper part of the figure from the * and the 

 right half of the oesophagus restored. In the middle of the apical organ is seen a small 

 group of otolith cells lying partly imbedded in the epithelium. '75/j. 



— 3. Vertical section, in the transversal jjlane, through the apical organ and oesophagus of an 



embryo in the III. stage. (Comp. Fig. 10). The canal marked e. s. is the upper part of the 

 entodermal sac. '75',. 



— 4. Vertical section through the oesophagus (in the transversal plane) of an embryo in stage II 



(corresponding to PI. III. Fig. 8). ent. = the median wall of the entodermal sac. ^-to/j. 



— 5. Vertical section through the excretory opening of a grown specimen. '*°/i. 



— 6. Vertical section ^in the transverse plane) of the apical organ of an embryo in stage II. 



The otolith mass is seen to be attached to a tuft of cilia, probably a rudimentary balancer. 

 The zone of cells at the upper edge of the organ bears long cilia, which are to form the 

 cupule. The mesodermal tissue is not yet developed. The endothelial membrane below the 

 organ is the dorsal wall of the infundibulum. Otoliths are seen developing within some 

 of the cells of the apical organ. ''^°/]. 



— 7—8. Horizontal sections (slightly oblique) through the apical organ of an embryo in Stage III. 



Fig. 7 at the level of the polar fields, fig. 8 below the polar fields. Outside the polar fields 

 are .seen the excretory canals. '75/,. 



— 9. Part of section through a genital organ, sliowing the discharging of the spermatozoa into 



the cavity of the genital organ. ^°°/j . 



— 10. \'ertical section (in the sagittal plane) through the apical organ of an embryo in Stage III; 



showing the excretory canals. (Comp. Fig. 3). '7?/, . 



— II. Vertical section, parallel to the sagittal plane, through the ectodermal invagination over the 



genital organ. The entodermal lining of the genital organ was highly vacuolated; the 

 vacuoles have been omitted in the figure. Some of the packets in the testis are seen to 

 contain ripe spermatozoa. '* ', . 



