364 SYDNEY J. HICKSON. 



I have examined the ectoderm very carefully, to determine 

 whether it is ciliated or not ; but neither in the living animal 

 nor in my best preparations can I find any trace of cilia on 

 any of the cells, eitlier on the general surface of the body, the 

 extensible portion of the body- wall, the tentacles, or the oral 

 disc. 



This is not inconsistent with the work of other observers, 

 for in no other work on the minute anatomy of Alcyonarians 

 can I find the external ectoderm^ described as being ciliated. 

 This is a noteworthy point, because in the Actinise, according 

 to the Hertwigs and other observers, the ectoderm is ciliated. 

 In the Hydrozoa the ectoderm is never ciliated, so that in this 

 respect the Alcyonaria apparently agree with the Hydrozoa 

 and differ from the sea-aneraones. 



It is difficult to determine with certainty the origin of the 

 cells that give rise to the spicules ; but, for many reasons, I 

 am inclined to agree with von Koch's (23) results on Gorgonia 

 and Clavularia, and attribute them entirely to the ectoderm. 

 Among the interstitial cells of this tissue one frequently finds 

 large spherical cells which lie beneath their neighbours, and 

 cells very similar to these maybe seen isolated in the subjacent 

 mesogloea. Moreover large cavities may be seen in isolated 

 cells in the same region, which, in all probability, contained a 

 spicule before decalcification (fig. 17, sp. c). 



Attention has already been directed to the fact that the 

 spicules are far more numerous at the periphery than in the 

 deeper parts of the colony. This suggests very forcibly that 

 the spicules are only formed at the periphery, and that with 

 the growth of the mesogloea they become more and more sepa- 

 rated from one another. It is not likely that during the growth 

 of the colony they are dissolved. 



* Von Koch (23) says that in the Alcyonaria the ectoderm-cells are 

 provided with cilia, which often disappear iu old colonies; but he gives 

 no figure to illustrate this ciliation. KoUiker (24) says that the ectoderm 

 of Pennatulids shows ciliation, but he was unable to determine its extent 

 in the specimens he examined, and Prof, Marshall did not describe nor 

 figure it. 



