XXX INTRODUCTION. 



its inner surface with regularly recurring serrations, which 

 are the optical expression of that peculiar armature to be 

 described presently. 



Under the stimulus of pressure, when subjected to micro- 

 scopical examination, and doubtless under nervous stimulus, 

 subject to the control of the will, during the natural exer- 

 cise of the animal's functions, the cnida suddenly emit 

 their contents with great force, in a regular and prescribed 

 manner. It must not be supposed, however, that the pres- 

 sure spoken of is the immediate mechanical cause of the 

 emission : the contact of the glass-plates of the compres- 

 sorium is never so absolute as to exert the least direct force 

 upon the walls of the capsule itself; but the disturbance 

 produced by the compression of the surrounding tissues 

 excites an irritability which evidently resides in a very 

 high degree in the interior of the cnidce ; and the pro- 

 jection of the contents is the result of a vital force. 



In general the eye can scarcely, or not at all, follow the 

 lightning-like rapidity with which the chamber and its 

 twining thread are shot forth from the larger end of the 

 cnida. But sometimes impediments delay the emission, 

 or allow it to proceed only in a fitful manner, a minute 

 portion at a time ; and sometimes, from the resistance of 

 friction (as against the glass-plate of the compressorium), 

 the elongation of the thread proceeds evenly, but so slowly 

 as to be watched with the utmost ease ; and sometimes the 

 process, which has reached a certain point normally, be- 

 comes, from some cause, arrested, and the contents of the 

 cell remain permanently fixed in a transition state. Thus 

 a long continued course of patient observation is pretty 

 sure to present some fortuitous combinations, and abnormal 

 conditions, which greatly elucidate phenomena that nor- 

 mally seemed to defy investigation. 



In watching any particular cnida, the moment of its 

 emission may be predicted with tolerable accuracy by the 

 protrusion of a nipple -shaped wart from the anterior 

 extremity. This is the base of the thread. The process 

 of its protrusion is often slow and gradual, until it has 

 attained a length about equal to twice its own diameter, 

 when it suddenly yields, and the contents of the cnida dart 

 forth. At this instant I have, in many instances, heard a 

 distinct crack or crepitation, in the examination of cnidce 

 both of this species and of S. parasitica. 



