590 CET^SSWELL SHEAEEE. 



on the external surface are not shown, for the sake of clearness. The 

 head-kidney cell is seen on the ventral side of the oesophagus. In the 

 apical region some ectoniesoblast cells are shown. 



Fig. 10.— Slightly older stage than that of the last figure. The head- 

 kidney is represented by a string of three cells. 



Fig. 11. — External view of a late gastrula of Eupomatus showing 

 the portion of the blastopore that remains as part of the mouth. 



Fig. 12. — Still later stage than that shown in fig. 10. This stage is 

 about thirty-six hours old. 



Fig. 13. — Still later stage than the last. 



Fig. 14. — Anal end of a young trochophore of Euj)omatus showing 

 the double formation of the anal vesicle. 



Fig. 15. — Early trochophore of Eupomatus older than that of 

 fig. 13. 



Fig. 16. — Early trochophore of Eupomatus forty-eight hours old. 

 The ccelomesoblast cell is seen in the wall of the gut above the anal 

 aperture. 



PLATE 23. 



Fig. 17. — Hydroides norvegica. The trochophore in this figure 

 is i-epresented as tilted up and seen from the oral surface. The ccelo- 

 mesoblast is seen arising from two cells in the gut- wall dorsal to the 

 anal vesicle. 



Fig. 18. — Hydroides norvegica. The lowerportion of the trocho- 

 l>hore is shown under high magnification and slightly compressed under 

 the cover-glass. The opening of the head-kidneys into the proctodteum 

 is shown, and the ccelomesoblast. 



