the british - Corning Museum of Glass

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 microscopical
examination, and doubtless under nervous stimulus,
subject to the control of the will, during the natural exercise
of the animal's functions, the cnidœ suddenly emit
their contents with great force, in a regular and prescribed
manner. It must not be supposed, however, that the pressure
spoken of is the immediate mechanical cause of the
emission: the contact of the glass-plates of the compressorium
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 cnidœ; and the projection
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, becomes,
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 normally
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 cnidœ
both of this species and of S. parasitica.