258 J. H. ASH WORTH. 



spicules from the tentacles and distal portion of the polyp 

 renders the ectoderm softer and more pliable, and therefore 

 more readily acted upon by the contraction and expansion of 

 the muscle processes of its cells. 



Among the ordinary columnar cells there are in the ectoderm 

 of the stem and the body of the polyp large swollen cells 

 which probably secrete the mucus which thinly covers the 

 external surface of most of these parts (PL 26, fig. 17, 

 Muc. C). These mucus-cells are large and abundant in the 

 angle of the Y-shaped piece formed where a stem divides. 



The ectoderm cells present on their outer side a moderately 

 plane surface, but on the inner side a more irregular one, as 

 numerous processes pass from the cells inwards and establish 

 communication either with the eudoderm or with cells lying 

 deeper in the mesogloea (fig. 17). 



On reaching the base of the colony the ectoderm curves 

 inward, and was applied to the face of the rock to which the 

 colony was attached. 



The stomodaeum is ectodermic in origin in this as in other 

 Alcyonaria (Wilson, 1883), as may be seen from a study of 

 the buds. The presence of a ventral ciliated groove and of 

 gland-cells and other features of its structure have already been 

 referred to (p. 251). The ectoderm of the stomodaeum and the 

 adjacent endoderm are connected by numerous cells or strands 

 of cells passing through the thin raesogloeal lamina which 

 separates the two cell layers (PI. 26, fig. 18). 



The ectoderm cells give rise to nematocysts and spicules. 



Nematocysts. — The nematocysts are exceedingly numerous 

 in the ectoderm of the tentacles and pinnules (PI. 25, fig. 11, 

 Neni.) ', there are large numbers in the ectoderm of the body of 

 the polyp, rather fewer in the ectoderm of the stem, and a few 

 in the ectoderm of the oral disc, in the funnel leading to the 

 mouth, and also in the upper part of the stomodaeum. The 

 nematocysts are, as in other Alcyonaria, exceedingly small, 

 their length being -008 mm. and their breadth '002 mm. to "003 

 mm. They have bluntly pointed ends and are circular in trans- 

 verse section. Each nematocyst is formed in a cnidoblast cell 



