XXXYl 
INTRODUCTION. 
lining membrane of high contractile power, lessened, on 
irritation, the volume of the cavity, and forced out the 
wire. 
The cnida is filled, however, with a fluid. This is very 
distinctly seen, occupying the ca^dty, when from any im- 
pediment, such as above described, the wire flies out 
fitfully — waves, and similar motions, passing from wall to 
wall : sometimes, even before any portion of the wire has 
escaped, the whole mass of tangled coils is seen to move 
irregularly from side to side, within the capsule, from the 
operation of some intestine cause. The emission itself is a, 
process of injection ; for I have many times seen floating 
atoms driven forcibly along the interior of the ecthoresum, 
sometimes swiftly, and sometimes more deliberately. 
Nothing that I have seen, would lead me to conclude that 
the wall of the cnida is ciliated. 
I consider, then, that this fluid, holding organic cor- 
puscles in suspension, is endowed with a high degree of 
expansibility ; that, in the state of repose, it is in a con- 
dition of compression, by the inversion of the ecthorceum ; 
and that, on the excitement of a suitable stimulus, it 
forcibly exerts its expansile power, distending, and con- 
sequently projecting, the tubular ecthorceum, — the only part 
of the wall that will yield without actual rupture. 
The cnidce cannot, I think, be regarded in the light of 
cells, since they are but the contents of other vesicles, 
which thus present a higher claim to the character of cell- 
wall. In the craspeda of S. parasitica, may be seen many 
of the chambered cnidce, bearing this outer envelope, 
which, without determining anything concerning its nature, 
I shall distinguish as peribola. Many of the cnidce have 
ruptured their investing membrane, which gives way at no 
special point, sometimes at the anterior end, sometimes at 
the posterior, and as frequently, all down the side. The 
perihola thus ruptured, may be seen in many instances still 
hanging about the cnida, while others are quite free from 
any remains of it, and in some cases I have seen the cnida 
still enveloped in its perihola, unruptm’ed. 
The perihola I have seen investing, and hanging around 
the cnidce of the spiral and glohate kinds, and this circum- 
stance has afforded me an additional ground for presuming the 
latter to belong to this category of organs (figs. 11, 12, ^). 
It appears necessary that the cnida should set itself free 
