XXX 
INTEODL’CTIOX. 
its inner surface with regularly recurring seiTations, 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 aniinal’s functions, the cnidce 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 sm-rounding 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 tlie 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 Avith 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 tAvice its OAvn diameter, 
Avhen 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 8. parasitica. 
