THE STRUCTURE OF XENIA HICKSONI. 189 



The siphonozooids which occur in some other Alcyonacea {e.g., Sarco- 

 phyton) and in Pennatulids are the only recorded examples of polyps 

 in which the ventral and lateral mesenterial filaments are absent. 

 According to Wilson (1884), these siphonozooids derive their food 

 supply from the autozooids or feeding polyps, the dorsal mesenterial 

 filaments of the former creating upward currents which cause a flow of 

 fluid from the autozoids to the siphonozooids, through the canals which 

 connect them together. Food undergoes digestion in the autozooids, 

 and some of the products are passed on to the siphonozooids, hence the 

 latter do not require cells to produce a digestive secretion. 



In this Xenia the secretion in connection with the digestive cavity 

 is formed, not by endoderm cells, but by cells which are derived from 

 the ectoderm, as from a study of the buds I have found that the 

 stomodseum is ectodermic in origin in this as it is in other Alcyonaria 

 (Wilson, 1883). Since the absence of ventral mesenterial filaments in 

 Xenia Hicksoni was proved I have carefully examined all the other 

 specimens of Xenia at my disposal. These are sixteen in number, viz., 

 one from Talisse Island, North Celebes, one from Rotuma, Fiji Islands, 

 and fourteen from various reefs in the Pacific. In all these the ventral 

 and lateral mesenterial filaments are absent, there being only a very 

 slight thickening of the free edges of the mesenteries, this thickening 

 being entirely due to a slight increase in the amount of the mesogloea 

 along the free edges. In all the specimens the two dorsal mesenterial 

 filaments are present and have the typical course and structure. In 

 Heteroxenia Elizabeth.ee the two dorsal mesenterial filaments only are 

 present. The absence of ventral and lateral mesenterial filaments 

 may, therefore, be considered as one of the characters which distinguish 

 the polyps of the genera Xenia and Heteroxenia from those of other 

 Alcyonaria. In two at least of the specimens of Xenia 1 from Dr. 

 Willey's collection some of the cells of the stomodseum are goblet-like, 

 and similar to those described above in the stomodseum of Xenia 

 Hicksoni. 



Coelentera of Polyps. — The other parts of the colony present a 

 structure similar, in the main features, to that of other 

 Alcyonaria. The polyps are, for the greater part of their length, 

 bound together in bundles of about forty to sixty, each bundle 



1 X. crassa, Schenk, and X, viridis, Schenk. 



