NETTLING CELLS. 197 



Nothing is more wonderful than the structure of 

 this thread, when we examine a minute portion of its 

 length by the aid of the microscope. It is made up 

 of millions of transparent oval or oblong sacs, bags of 

 clear, tough membrane closed at each end, and of 

 such dimensions that five hundred of them, if placed 

 in contact end to end, and three thousand, if laid side 

 by side, would lie within the length of an inch. 

 Within each capsule is seen a wire coiled up loosely, 

 and thus occupying the interior. 



Such is the armature as it lies ready for action. 

 But, on the stimulus of the animal will, the oval 

 capsules project from the periphery of the containing 

 thread by thousands, and instantly each shoots its 

 shaft. In other words, the coiled-up wire is evolved 

 with lightning-like rapidity from the smaller end; 

 and that not by direct projection, but by eversion ; 

 for, wonderful to tell, the wire is tubular throughout, 

 and in order to be shot it must be completely turned 

 inside-out, just as you invert a stocking. 



Well, you see this subtle wire running out, evolv- 

 ing its inner surface as it goes, so quickly that it is 

 only under rare circumstances that the eye can follow 

 the process. But now you discern that even this 

 wire is not a simple tube. For throughout its entire 

 length a length which often reaches to thirty times 

 that of its capsule it is ridged with one, two, or 

 three elevated carince, or thickened bands, which run 

 round it in regular spiral turns, at exactly prescribed 

 distances, and at an exactly prescribed angle of in- 



