422 EVENINGS AT THE laCEOSCOPE. 



distinctly visible. An inexperienced observer would 

 be liable, under such circumstances, to suppose that 

 the screw, when formed of a single band, as in T. cras- 

 sicornis, is itself the wire ; an error into which I had 

 myself formerly fallen. An error of another kind I fell 

 into, in supposing that the triple screw of the wire in 

 0. Smithii was a series of overlapping plates: the 

 structure of the armature is the same in all cases (with 

 the variations in detail that I have just indicated) ; and 

 the structure is, I am now well assured, a spiral thick- 

 ened band running round the wall of the ecthoroBum 

 on its exterior surface. I have been able when exam- 

 ining such large forms as those of Corynactis viridis 

 and Oyathina Smithii, with a power of 750 diameters, 

 to follow the course of the screw, as it alternately ap- 

 proached and receded from the eye, by altering the 

 focus of the object-glass, so as to bring each part suc- 

 cessively into the sphere of vision. 



" These thickened spiral bands afford an insertion 

 for a series of firm bristles, which appear to have a 

 broad base and to taper to a point. Their length I 

 cannot determinately indicate, but I have traced it to 

 an extent which considerably exceeds the diameter of 

 the ecthorcEum. These barbed bristles are denominated 

 pterygia. 



" The number of pterygia appears to vary within 

 slight limits. As well as I have been able to make 

 out, there are but eight in a single volution of the one- 

 banded strebla in T. crassicornis ; while in the more 

 complex screws of S. parasitica, Oor. viridis, and Gy. 

 Smithii, there appeared to be twelve in each volution. 



" The barbs, when they first appear, inyariahlj pro- 

 ject in a diagonal direction from the ecthorwum; and 



