268 Cc. H. MARTIN. 
posterior end in the neighbourhood of the Hydra, and everts 
its pharynx to its full extent. 
The Microstoma then swims over the surface of the Hydra, 
usually attacking the lower part of its body with its pharynx 
fully everted (vide fig. 10). The Hydra then usually becomes 
strongly contracted, and sweeps its tentacles over to the side 
on which it has been attacked, though under these conditions 
the tentacles do not grasp the Microstoma, but remain ex- 
tended almost parallel with its body, and it would appear as 
though the pharyngeal secretion had a paralysing action on 
the Hydra. In many cases, after a time, the Microstoma 
leaves its prey, and in such a case the Hydra does not seem 
much the worse for the attack, but if the Hydra is of small 
size, it may be engulfed, and swallowed whole. 
To further examine the effect of this pharyngeal secretion 
I placed some Hydra, which had been kept living in a solution 
of neutralroth, in a watch-glass with a Microstoma, and it 
could then be seen that the vacuoles of the ectoderm of the 
Hydra, which had been stained a pink colour by the neutral- 
roth, took a yellowish-brown colour under the action of the 
digestive fluid, indicating that the secretion was probably of 
an alkaline nature, and possibly allied to trypsine. If a 
Microstoma which contains nematocysts is placed in a watch- 
glass, and a dilute acid or alkaline added, the nematocysts 
under the skin are discharged. It would be very interesting — 
to determine how far the Microstoma is capable of using its 
nematocysts for purposes of attack or defence. 
One of the common enemies of Microstama appears to 
be Cheetogaster, which devours it greedily. One evening 
I placed six large Microstoma in a watch-glass with two 
Cheetogaster, and by 10 a.m. next morning all had been 
eaten. In one case which I examined-I found a Cheto- 
gaster, some time after it had swallowed three Microstoma, 
had eight large exploded nematocysts in its gut, but the 
presence of nematocysts in its prey seems to have no deter- 
rent effect upon the Cheetogaster. 
As regards the mechanism by which this discharge is 
