REPORT ON THE XENIIDAE COLLECTED BY DR WILLEY. 525 



In the same sections, among the ordinary columnar cells which line the greater part 

 of the stomodaeum, there are swollen and apparently empty cells similar in appearance 

 and position to those described in X. Hicksoni (1899, p. 251). They are cells which have 

 been swollen by some secretion to which they give rise, and, having discharged this 

 secretion, now appear empty. The stomodaeum of the stained polyps of X. crassa 

 also shows, when the walls are seen by transparency, numerous small light areas, 

 which probably indicate the presence of these secretory cells. The polj'ps of X. uni- 

 hellata and X. membranacea are not sufficiently well preserved to show this point in 

 their structure. 



In all the species the polyps are devoid of ventral and lateral mesenterial 

 filaments but the dorsal mesenterial filaments are present and well developed. They 

 run in a sinuous course down the dorsal side of the coelenteron and in the primary 

 polyps extend to the base of the colony. (See Fig. 7.) They agree in structure with 

 those of X. Hicksoni. 



Coelentera of Polyps. 



The eight mesenteries are arranged as in typical Alcyonaria. The mesenteries of all 

 the species in the collection are very thin, much thinner than those of X. Hicksoni, and 

 their retractor muscles are feebly developed, forming only a very slight ridge on the 

 ventral face of each mesentery. The small size of these muscles probably accounts for 

 the non-retractile character of the polyps. 



Between the two endodermic lamellae covering the mesentery there is a thin plate 

 of mesogloea which is slightly thickened near the inner or free edge of the mesentery. 

 In this portion cells are usually present similar to those found in a corresponding 

 position in the mesenteries of X. Hicksoni. These are endoderm cells which have 

 migrated into the mesogloea and give rise to the genital products. They have reticulate 

 protoplasm and large nuclei. (See Fig. 14.) 



Mesogloea, its Canals and Cells. 



As in X. Hicksoni, there is around each coelenteron in the stem (at any rate in 

 its upper part) a denser, more deeply staining cylinder of mesogloea which belongs to the 

 coelenteron within it. (See Fig. 7.) This is almost free from cells but is surrounded 

 by a cylinder of ectoderm cells in which there are numerous spicules. (1899, PI. 25, 

 Figs. 8, 9.) Spicules have migrated from this chain of cells into other parts of the 

 mesogloea, in which they are more abundant than in X. Hicksoni. 



The superficial canal system is very uniform throughout, consisting of a plexus of 

 canals situated about '1 mm. below the ectoderm of the stem. (See Fig. 9.) The outer 

 wall of these canals is thicker than the inner owing to the cells of the former 

 being more columiuir than those of the latter wall. The cavity of this system of 

 canals is invaded by zooxanthellae throughout. 



The internal canal system varies in the development of its parts to a considerable 

 extent. The extreme in one direction is reached in X. viridis, in which the longitudinal 

 canals which run in a sinuous course in the mesogloea about midway between the 

 coelentera are feebly marked. As if to compensate for this the transverse canals which 



w. IV. 70 



