XXXVI 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 cavity, 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 ecthorceum, 

 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 8. parasitica, may be seen many 

 of the chambered cnidce, bearing this outer envelope, 

 which, without determining anything concerning its nature, 

 I shall distinguish as the peribola. Many of the cnida 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 

 peribola 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 peribola, unruptured. 



The peribola I have seen investing, and hanging around 

 the cnidce of the spiral and globate 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, g). 



It appears necessary that the cnida should set itself free 



