56i SYDNEY J. HICKSON. 



vertically and closely approximated, give the appearance of a 

 columnar epithelium. It is impossible to say from the study 

 of non-living material only whether these cells are really 

 ciliated, but I am inclined to think that they are not, as in 

 some places I could distinguish a delicate membrane, like a 

 thin cuticle, covering the free edges of these cells. In the 

 above description of the histology of this portion of the ecto- 

 derm, I have described what I believe to be the true nature of 

 it after a careful examination of numerous sections; but as the 

 cells are here so very small, a renewed examination of speci- 

 mens specially preserved for histology is desirable. 



The ectoderm covering the tentacles is composed of two or 

 three rows of cells the most superficial of which is distinctly 

 ciliated. These cilia are probably in the living animal long 

 and powerful and produce currents by their action which bring 

 food to the polype. 



The mesoderm consists of a homogeneous matrix in which 

 may be found cells and fibres. The cells are usually pyra- 

 midal in shape but sometimes spherical or bipolar (fig. 12, c). 

 The angles of the cells are usually drawn out into long pro- 

 cesses lying in the matrix. Fibres are seen spreading through 

 the matrix in various directions just as described and figured 

 by Kolliker (11) in other Alcyonarians. In the actively 

 growing mesoderm, such as is found in the young platforms, 

 groups of small cells may be seen, which, budding off" from the 

 ectoderm, sink into matrix of the mesoderm. These groups of 

 cells give rise to the spicules in the following manner. At 

 first a small calcareous particle is seen lying in the midst of 

 these cells, and as this increases in size the cells become more 

 and more flattened around it until only a delicate membrane 

 with two or three nuclei can be seen covering the spicule. 

 After a time even this membrane disappears and the spicule 

 lies freely in the matrix. 



The endoderm consists of a layer of loose spherical cells 

 varying considerably in size and appearance (fig. 12, Ep^). 

 The cells which lie next to the mesoderm are the smallest and 

 youngest, and they stain well in hacmatoxylin and borax 



