THE STRUCTURE OP XENIA HICKSONI. 231 



Two of them are crowded with sperm sacs (S.S.), many of which, owing to 

 mutual pressure, have lost their original spherical shape and are pentagonal 

 or hexagonal in section. The other coelenteron contained a similar number of 

 sperm sacs, but they have been omitted in order to more clearly show the 

 dorsal mesenterial filament (D.M.F.) and the thin edge of the ventral mesen- 

 tery (V. 31.). The superficial (Sup. Can.) and longitudinal (Long. Can. ) canal 

 systems, and their relation to each other and to the coelentera; the denser 

 cylinder of mesogloea (Mg. D. ) with its surrounding ectoderm cells enclosing 

 each coelenteron ; and the distribution of spicules, nematocysts, and zooxan- 

 thellse are also shown, x 35. 



Fig. 9. — A thinner transverse section through the upper part of one of the 

 stems. The coelentera in the peripheral part are smaller than those nearer the 

 centre. The coelenteron of a very young polyp is seen on the right. The cells 

 in the mesogloea of the mesenteries and sperm sacs in various stages of develop- 

 ment, attached to the mesenteries of the older coelentera, are also shown. The 

 number of flagella cut across in one section is seen; thus an idea may be 

 formed of their abundance and of their size relative to the endoderm and to 

 the coelentera. Many of the features to which attention was drawn in the 

 description of the previous figure are also shown here. X 50. 



Fig. 10. — Transverse section through a polyp at the level of the lower third 

 of the stomodseurn. The chief features shown are the gland-cells and 

 siphonoglyph of the stomodasum, the somewhat feebly developed retractor 

 muscles on the mesenteries, and the distribution of the spicules, nematocysts, 

 and zooxanthellas. X 50. 



Fig. 11. — Transverse section of the outer wall of a pinnule showing the 

 ectoderm containing nematocysts, the mesogloea, and the endoderm cells with 

 their reticulate protoplasm, x 800. 



Fig. 12. — A nematocyst in its cnidoblast cell, from a section. One half of 

 each coil of the thread in the upper part of the nematocyst was cut away in 

 sectioning, x 2000. 



Fig. 13. — Eight large spicules selected from those in the base of the 

 colony. Each spicule is situated in a small cavity in the mesogloea. The 

 nucleus and remains of the protoplasm of the spicule forming cell are seen. 

 From sections. x800. 



Fig. 14. — A portion of the outer part of the mesogloea from a transverse 

 section of a polyp. Four of the spicules present their edge and two their 

 flat face to the observer, x 800. 



Fig. 15. — Two very young spicules in their respective cells. The spicule 

 in A is 3£ f^ long and 2\ /* broad. The spicule in B presents its edge to the 

 observer. It is 1 ft long and 1 \ p thick. x 1000. 



Plate XXI. 



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

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

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



