THE STRUCTURE OF XENIA HICKSONI. 303 



PLATE 26. 



Fig. 16. — Section of the wall of a polyp which has passed very obliquely 

 through the mesogloea, to show the nervous system. Fine nerve fibrils in the 

 niesoglcea are connected on the outer side with cells in the outer part of the 

 mesoglcea, some of which send processes into the ectoderm. On the inner 

 side, the fibrils pass into small stellate nerve-cells situated just outside the 

 muscle processes of the endoderm cells, x 300. 



Fig. 17. — Longitudinal section through the body-wall of a polyp, to show 

 the general character of the ectoderm and endoderm cells, and also the 

 processes of ectoderm cells which penetrate the mesoglcea and establish 

 connection with the endoderm. x 400. 



Fig. 18. — Longitudinal section of the ventro-lateral portion of the lower 

 third of the stomodseum. The chief feature shown is the large gland-cells, 

 some of which have discharged their contents and appear empty, while others 

 are in the act of discharging their secretion, x 600. 



Fig. 19. — Transverse section of the end of a dorsal mesentery showing the 

 V-shaped mesenterial filament bearing cilia. X 600. 



Fig. 20. — Transverse section of the end of a ventro-lateral mesentery to 

 show the general character of the endoderm cells, the giant flagellum of one of 

 them, and the cells in the mesoglcea. x 600. 



Fig. 21. — Transverse section of the end of the same mesentery taken 

 '25 mni. higher, showing three fiagella occurring close together, x 600. 



Figs. 22, 23, 24. — Three fiagella from sections of polyps to show their 

 various degrees of flexion, x 600. 



Fig. 25. — Two abnormal forms of fiagella from sections. A from the 

 tentacle of a polyp, B from the portion of a ccelenterou in a stem, x 600. 



Fig. 26. — Endodermic myo-epithelial cells from teased preparations. In 

 connection with the one on the right there is a small stellate cell with three 

 long processes. This is probably one of the nerve-cells of the endodermic 

 portion of the nerve plexus, x 500. 



Fig. 27. — Isolated cells bearing fiagella, from teased preparations, x 500. 



PLATE 27. 



Fig. 28. — Section of a stem at the edge of the umbellate summit, to show 

 the formation of a young polyp "43 mm. long (II in table, p. 284). Note 

 that the endoderm of the free portion of the polyp is thick, while that of the 

 inner part of the ccelenteron is thin. The neighbouring ccp.lentera are cut very 

 obliquely, x 50. 



Fig. 29.— The superficial canal indicated by the asterisk in Fig. 9 enlarged. 

 The cells of the outer wall are longer and more columnar than those of the 

 inner wall, x 500. 



