XIV 
THE VOYAGE OF H.M.S. CHALLENGER. 
in the ectoderm of the foot-disc of Hydra. The remarkable development of pseudo- 
podia by the sarcostyles will be described below when considering the structure of those 
bodies. 
Thread-cells . — Of all the elements of the ectoderm the thread-cells or urticating 
capsules are the most characteristic. These bodies, however, are by no means confined to 
the Hydroida, but occur under various forms as a distinguishing and universally present 
feature in the whole of the Coelenterata with the exception of the sponges — taking for 
granted that these have their true place in the Coelenterate subkingdom. 
The thread-cell varies both in form and in size in different species of Hydroida, and 
even in different parts of the same Hy droid, but it may be described as essentially 
consisting of a containing capsule and a contained filament which under certain 
conditions admits of being projected from the capsule. 
The thread-cell however possesses a really complicated structure, the exact determina- 
tion of which is exceedingly difficult, but from what has been satisfactorily established by 
the most careful observations, it may be regarded as certain that the thread-cell in its 
typical form, such as is seen in the larger thread-cells which occur in the body of Hydra 
or in the spherical capitula which terminate the tentacles of Coryne, consists essentially 
of an oviform rigid capsule lined by a fine membrane, which at one end of the long 
diameter of the capsule is invaginated into its cavity in the form of a tube occupying 
the axis, and there becoming continuous with a very fine and long tubular filament 
which lies in a congeries of coils within the capsule. The whole is included in 
an external cell, the cnidocyst, within which it has been developed, while the cnidocyst 
itself sends off from a point close to its summit or discharging pole a minute bristle- 
like process of its protoplasm, the cnidocil, which projects beyond the surface of the ecto- 
derm into the surrounding water. 
This is the condition of the typical thread-cell in its quiescent state, but under the 
influence of certain stimuli, not yet well understood, and which are probably exerted through 
the medium of the cnidocil, its characteristic action is brought into play, and a remarkable 
change takes place in it. The tube, which in the quiescent state lay in the axis of the 
capsule, is now suddenly projected through the discharging pole, and this act is 
immediately followed by a similar projection of the fine tubular filament with which at 
its free end the axile tube is continuous, and the capsule is thus emptied of its contents. 
The whole of this process consists in an act of evagination by which the wider axile tube 
and the fine tubular filament into which this is continued are transferred from the inside 
to the outside of the capsule. 
When this sudden act of evagination has been completed it is seen that the portion 
of the tube which in the inverted state had lain in the axis of the capsule, is provided, in 
the form of thread-cell here taken as the type, with a verticil of barb-like spines attached 
to what has now become the outer surface of its walls. It continues to be attached by 
