368 EVENINGS AT THE MICROSCOPE. 



case the spiral runs from the east towards the north, sup- 

 posing the axis to point perpendicularly upwards. 



Sometimes, especially after having been expelled for 

 some time, the wall of the ecthoreum becomes so attenuated 

 as to be evanescent, while the strebla is still 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. crassicornis, 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 (7, Smithii was a series of 

 overlapping plates : the structure of the weapon is the 

 same in all cases (with the variations in detail that I have 

 just indicated) ; and the structure is, I am now well as- 

 sured, a spiral thickened band running round the wall of 

 the ecthoreum on its exterior . surface. I have been able, 

 when examining such large forms as these of Corynactis 

 viridis and Caryophyllia Smithii, with a power of 750 

 diameters, to follow the course of the screw, as it alter- 

 nately approached 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 determi- 

 nately indicate, but I have traced it to an extent which 

 considerably exceeds the diameter of the ecthoreum. 

 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 

 8. parasitica, Cor. viridis, and Car. Smithii, there appear 

 to be twelve in each volution. 



The barbs, when they first appear, invariably project in 

 a diagonal direction from the ecthoreum ; and sometimes 



