414 EVENINGS AT THE MICROSCOPE 



wire; an error into which I had myself formerly fallen. 

 An error of another kind 1 fell into, in supposing that the 

 triple screw of the wire in C. Smithii was a series of over- 

 lapping 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, 1 am now well assured, 

 a spiral thickened band running round the wall of the 

 ecthorceum on its exterior surface. I have been able when 

 examining such large forms as those of Corynactis viridis 

 and Cyathina Smithii, with a power of 750 diameters, to 

 follow the course of the screw, as it alternately approached 

 and receded from the eye, by altering the focus of the 

 object glass, so as to bring each part successively 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 de- 

 terminately 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 8. 

 parasitica, Cor. viridis, and Cy. Smithii, there appeared to 

 be twelve in each volution. 



"The barbs, when they first appear, invariably project 

 in a diagonal direction from the ecthorceum; and sometimes 

 they maintain this posture. But more commonly, either 

 in an instant, or slowly and gradually, they assume a 

 reverted direction. 



"From some delicate observations made with a very 



