No. 420.] HISTORY OF STICHOSTEMMA. 989 
'THIGMOTAXIS. 
The thigmotactic reaction is positive and distinct. The 
animal shows a strong tendency to crawl into interstices and 
is usually found in masses of algae. In clear water it will com- 
: monly select the angle between bottom and sides of the jar 
and gradually come to rest there. 
Several specimens in a jar of clear water will often aggre- 
gate in a single mass, crawling over and between each other 
and finally becoming nearly quiet. 
A single individual, when it changes its course of movement, 
does not commonly make.a curve but turns the head directly 
back against its own body and retraces its course. Undoubt- 
edly the animal reacts thigmotactically to its own body. Ina 
similar manner on coming into contact with the body of another 
individual, or indeed any object, the worm crawls along its 
sides or edges, seeking the spots where the greatest portion of 
its body is in contact with surfaces. 
FORMATION OF **CvsTS." 
Since the formation of the cyst-like slime covering, mentioned 
above, appears to be more or less closely connected with the 
thigmotactic reactions of the animal, it is discussed a little 
more fully here. 
As noted above, there is a considerable increase in the 
amount of slime secreted when the animal is irritated in any 
way. Any strong stimulation, e.g., strong light, handling in 
a pipette, etc., appears to cause this increase in some degree. 
Animals placed in a jar of perfectly clear water without algæ 
or other places of concealment show a tendency to settle in 
accumulations of their own slime, and it appears, in some 
cases at least, that the secretion becomes more dense under 
these conditions. It often happens that within a day or two 
an animal becomes surrounded by a dense coat of somewhat 
elastic slime, which even after removal of the animal retains 
a more or less definite form. Within this slow movements 
may go on continuously. Sometimes the body is so closely 
