XXXll INTRODUCTION. 



error of another kind I fell into, in supposing that the 

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

 imbricate 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 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 and 

 Caryophyllia, 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 objective, 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 determin- 

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

 considerably exceeds the diameter of the ecthorasum. These 

 barbed bristles I denominate pterygia. (See fig. 7, p.) 



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. crass icornis ; while in the more complex screws of S. 

 parasitica, Cor. viridis, and Cary. Smithii there appear 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 good 

 light, I have reason to conclude that the strebla, and even 

 the pterygia, are continued on the attenuated portion of the 

 ecthorceum, perhaps throughout its length. In Corynactis 

 and Caryophyllia I have succeeded in tracing them up a 

 considerable distance. In the latter I saw the continuation 

 of all these bands, with their bristles; but the angle of 

 inclination had become nearly twice as acute as before, 

 being only 22° from the axis. The appearance of the 

 attenuate portion, as also of the base of the ventricose part, 

 is exactly that of a three-sided wire, twisted on itself; the 

 barbs projecting from the angles. 



(2.) Tangled Cnidce (Cnidce glomiferce). This form is 

 very generally distributed, and is mingled with the former 



