INTRODUCTION. XXXV 



have actually seen the unevolved portion of the ecthorceum 

 running out through the centre of the evolved ventricose 

 portion. But perhaps the most instructive and convincing 

 example of all was the following. One of the large tangled 

 cnidce of Corynactis viridis had shot about half of its wire 

 with rapidity, when a kind of twist, or " kink," occurred 

 against the nipple of the cnida, whereby the process was 

 suddenly arrested. The projectile force, however, continuing, 

 caused the impediment to yield, and minute portions of the 

 thread flew out, piecemeal, by fits and starts. By turning 

 the stage-screw I brought the extremity of the discharged 

 portion into view, and saw it slowly evolving, a little at a 

 time. Turning back to the cnida I saw the kink gradually 

 give way, and the whole of the tangled wire quickly flew 

 out through the nipple. I once more moved the stage, fol- 

 lowing up the ecthorceum, and presently found the true 

 extremity, and a large portion of the wire still inverted ; 

 slowly evolving indeed, but very distinct throughout its 

 whole course, within the walls of the evolved portion 

 (fig. 8). 



From all these observations, there cannot remain a doubt 

 of the successive eversion of the entire ecthorceum. It may 

 be asked, What is the nature of the force by which the 

 contained thread is expelled? That it is a potent force, 

 is obvious to any one who marks the sudden explosive 

 violence with which the nipple-like end of the cnida gives 

 way, and the contents burst forth ; as also the extreme 

 rapidity with which, ordinarily, the whole length is evolved. 

 A curious example of this force once excited my admiration : 

 the ecthorceum from a cnida of Corynactis viridis was in 

 course of rapid evolution, when the tip came full against 

 the side of another cnida already emptied. The evolution 

 was momentarily arrested, but the wall of the empty 

 capsule presently was seen to bend inward, and suddenly 

 to give way, the ecthorceum forcing itself in, and shooting 

 round and round the interior of the cnida. 



The most careful observations have failed to reveal a 

 lining membrane to the cnida. I have repeatedly dis- 

 cerned a double outline to the walls themselves — the 

 optical expression of their * diameter ; but have never 

 detected any, even the least, appearance of any tissue 

 starting from the walls, as the ecthorceum bursts out. My 

 first supposition, reluctantly resigned, was, that some such 



