THE NEMATOCYSTS OF TURBELLARIA. 267 
take up a wandering existence in the body cavity. Finally 
the nematocyst is transported to a position directly under the 
ectoderm ; here it lies in the vacuole (vide. figs. 2—4) (which 
is not an artifact, since it can be seen in the living animal) 
surrounded by about six cells. The wall of the vacuole after 
a time becomes thinner and denser. There is one point of 
great interest as regards the orientation of the nematocysts 
under the skin. The large barbed nematocysts in their final 
position, always lie so that the thread, when it is discharged, 
will pass out of the animal, although they may he pointing in 
any direction while they are still in the gut cells or the body 
cavity. This rule does not seem to hold good in the small 
cylindrical nematocysts, which, as far as I can see usually lie 
almost parallel to the surface. It is very difficult to say how 
such an orientation can be effected, but something of the same 
kind has been detected in A#olids, and I believe that the same 
difficulty is present in the nematocysts of the tentacles in 
Ceelenterates. In Hydra the tentacles are crowded with ripe 
nematocysts, but the chief region in which young nematoblasts 
are to be found is the distal region of the body below the 
level of the tentacles. 
As regards the mode of infection it was rather difficult 
& priori to see how such asmall animal as Microstoma could 
devour a Hydra; but it must be remembered that Hydra is 
generally observed in an extended condition, whereas Micro- 
stoma, except immediately after a meal, is never seen extended 
to its full size. Even the small Microstoma will readily 
swallow a fairly large Cyclops or Lycneeid, and these are more 
awkward animals to digest than a contracted Hydra rubra. 
If a fasting Microstoma is placed in a watch-glass which 
contains some small Hydra, it is almost certain in a short 
time to come into contact with one of them. If the Micro- 
stoma comes suddenly against the tentacles of the Hydra it 
contracts itself immediately, and in this condition it may fre- 
quently be killed by the discharged nematocysts. Asa rule, 
however, the Microstoma fixes itself for a short time by its 
