THE NEMATOCYSTS OF TURBELLARIA. 269 
effected I think that any muscular or nervous influence in the 
case of the nematocysts of Microstoma is excluded, and I have 
never found any trace of a cnidocil-like structure. I am 
inclined to agree with Grosvenor that the discharge of a ripe 
nematocyst is only effected when it is brought into a solution 
of less osmotic pressure. At the time when the nematocysts 
of the Hydra are ingested by the Microstoma, they lie in a 
mixture consisting of the pharyngeal secretion of the Micro- 
stoma and the partly digested ectoderm cells of the Hydra. 
When the Microstoma is irritated by acetic acid or an alkaline 
solution, the ectoderm probably becomes slightly contracted, 
and the end of the nematocyst is forced between the ectoderm 
cells, water is taken up, and an explosion follows. 
In order to establish a definite proof that the nematocysts 
of a Microstoma proceed from its prey, there seem to be only 
three courses open : 
(1) To feed the Microstoma upon Hydra in which the 
nematocysts have been stained intra vitam with some dye, 
and follow the passage of the coloured nematocysts through 
the gut under the ectoderm of the Microstoma. 
(2) To feed the Microstoma upon some other Ccelenterate 
which has nematocysts of a different shape to those of 
Hydra. . 
(3) To rear Microstoma from the egg without feeding it 
on Hydra. 
In the first case I kept some Hydra in water containing 
methylene blue, and found that some nematocysts, usually 
those that were not fully developed, took up the stain with 
great intensity. I fed some Microstoma on these Hydra, and 
a Microstoma which was placed with these Hydra at 1 o’clock 
had a mass of blue in its gut at 5 o’clock. Next day 
in most cases the colour had faded, and in only one case 
did I see blue nematocysts actually under the skin. This 
specimen was mounted, but unfortunately the colour has 
since, to a large extent, faded. 
With regard to the second course, this is the method that 
has been used with such success by Strethill Wright, 
