No. 420.] HISTORY OF STICHOSTEMMA. 981 
may remain for some weeks, either moving slowly or not at 
all, or it may leave in a few days. Left in jars of clear water 
without algae, the animals are likely to make these slime cov- 
erings repeatedly. As will appear below, I believe that the 
. fact that the animal is disinclined to leave the slime is a 
thigmotactic reaction. Violent stimulation will often induce 
removal, or the slime may be picked apart with needles, thus 
freeing the animal, which is active and normal in appearance 
in every respect. In many cases mutilated specimens or ani- 
mals in impure water form these coverings and remain in them 
until death. 
It is possible that the worms may form these “ cysts " about 
themselves during the winter. These do not, however, enable 
them to resist freezing and do not serve as a protection against 
desiccation, as will appear below. It is difficult to understand 
how the animal is better protected within this “ cyst,” except, 
perhaps, as regards predatory enemies, than it is under the 
usual conditions. While this process is perhaps to be regarded 
as showing the manner in which the process of encystment 
may arise, I think it is certain that in its present development 
this slime covering is of little value as a cyst. The causes 
and method of its formation are discussed below. 
MOVEMENTS. 
Locomotion forward is accomplished principally by means 
of the cilia with which the body is covered. During the for- 
ward movement the head is turned slightly from side to side 
in regular succession, and as the body follows the same course 
the movement as a whole is distinctly sinuous, the body show- 
ing altogether four or five curves to left and right at any given 
time. It is probable that under normal conditions the sinuous 
character of the movement is a distinct aid in locomotion. In 
crawling through masses of algz the lateral movements of the 
head will, of course, enable the animal to find the interstices 
more readily, while the sinuous bending of the body affords a 
firmer support and a greater power for forcing a way among the 
tangled threads. The value of these movements in enlarging 
