992 THE AMERICAN NATURALIST. [VOL. XXXV. 
to temporary excitement in consequence of handling, strong 
light, etc., when first introduced into the jar. For some time 
they are very active and many of them reach the angle 
between the surface of the water and the sides of the jar. 
Some of them force themselves into the angle with powerful 
movements and thus leave the water. Later the movements 
become less rapid, and even when they happen to reach the 
angle between the water and the side of the jar they content 
themselves with following it or remaining quietly in it. More- 
over, others have meanwhile come into contact with other indi- 
viduals or with slight accumulations of slime and tend to remain 
quiet. It is probable that there is also some degree of accli- 
matization to the new conditions, so that they in themselves 
fail after a time to stimulate the animal as strongly as they did 
at first. 
It is clear that the formation of these coverings cannot be 
regarded as constituting a true encystment. The animal does 
not enter a quiescent stage within the covering, nor is it enabled 
to survive conditions which would otherwise cause death, except 
that its life may be lengthened for a few moments by the 
formation of the covering after it leaves the water, or that it 
may in some cases escape the attack of predatory enemies by 
formation of the slime covering. 
The formation of the coverings is the result of the follow- 
ing conditions: the surface of the body is constantly secret- 
ing slime, and stimulation or irritation causes an increase in 
the amount of the secretion, perhaps also some change in 
the quality; and secondly, accumulations of this slime are of 
sufficient density to call forth in some degree the positive 
thigmotactic reaction. 
On the other hand, it is quite possible, and even probable, 
that the process of encystment might arise in some such man- 
ner as this in a positively thigmotactic animal. Indeed, in the 
present case a change in the quality of the secretion, rendering 
it less permeable to water, appears to be the only, or at least 
the principal, modification necessary for the formation of a 
true cyst. It is possible that in Stichostemma the secretion 
may become so modified in time that the animal will succeed 
