THE STRUCTURE OF XENIA HICKSONI. 1 93 



The stomodseum 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 stomodseum and the adjacent endoderm are connected 

 by numerous cells or strands of cells passing through the thin 

 mesogloeal lamina which separates the two cell layers (PI. XXI., 

 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. XX., Fig. 11, Nem.) ; 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 stomodseum. The nernatocysts are, as in other Alcyonaria, 

 exceedingly small, their length beiug *008 mm. and their breadth 002 

 mm. to "003 mm. They have bluntly pointed ends and are circular in 

 transverse section. Each nematocyst is formed in a cnidoblast cell 

 (PI. XX., Fig. 12, Cn.C), the nucleus and protoplasm of which lie 

 flattened against the capsule on one side and a little nearer the inner 

 than the outer end of the capsule. The filament or thread lies coiled 

 up inside this capsule, there being about twelve coils distinguishable 

 in the most favourable specimens. The thread appears to be quite 

 simple, there being no barbs visible. Its length when shot out would 

 probably be about 80 fji ("08 mm.). The nematocysts are usually 

 placed with their long axes at right angles to, and their bluntly pointed 

 ends level with or slightly projecting from, the free surface of the 

 ectoderm. This can be especially well seen in sections of the tentacles 

 and pinnules (see Fig. 11). The nematocysts stain deeply with 

 thionin, heematoxylin, and especially with iron hsematoxylin (Heiden- 

 hain). Iron hematoxylin is exceedingly useful for staining the small 

 nematocysts of Alcyonaria ; in fact, without some good staining 

 reagent it would be very difficult to find the minute capsules in many 

 cases. Moseley (1881, p. 1 19) wrote that " no nematocysts were found 

 in Sarcophyton," but by using the iron haematoxylin stain I have found 

 them in sections passing through the ectoderm of the tentacles. They 

 are very similar in shape to those of Xenia Hicksoni, but smaller in 



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