No. 420.] HISTORY OF STICHOSTEMMA. 991 
is clearly evident from the apparent reluctance noted above 
to leaving the covering, even after violent stimulation. The 
head being protruded finds no contact surfaces except the flat 
glass, and is again withdrawn. In some cases worms which 
had been removed from the mucous coverings happened to 
come into contact with the latter again in their wanderings, 
and after crawling about over them reéntered them, thus show- 
ing in the clearest manner possible the thigmotactic nature of 
their preference for these cyst-like coverings. 
Another interesting habit which these worms exhibit is 
closely related as regards its causes and nature to that just 
described, and is also clearly thigmotactic. In many cases, in 
their restless wanderings after introduction into jars of clear 
water, the worms reach the surface. They may crawl any- 
where upon the surface film, but prefer the margins, where the 
surface of the water curves slightly upward toward the glass. 
Undoubtedly the surface tension of the water affords a certain 
resistance to their movements here and calls forth a contact 
sensation. Into this angle between the surface film and the 
side of the jar the worms push their way and then crawl com- 
pletely out of the water and up the side of the jar. The slime 
secreted by the surface prevents immediate desiccation, but 
becomes thicker and harder as evaporation goes on, and thus 
in even greater degree than the slime coverings in the water 
affords the sensation of contact. The worms never attempt to 
return to the water, but remain within the cases thus formed 
until death ensues, which occurs within a few hours. The 
whole series of reactions leading to the emergence from the 
water and the failure to seek it again is thigmotactic. 
If the animals do not leave the water within the first twenty- 
four hours after being placed in the clear water they very rarely 
do it afterward unless they are again set in active motion by 
some strong stimulus. I have kept worms in a jar of this kind 
for three months ; the only specimens which left the water did 
so within the first day or two, although the conditions within 
the jar remained equally unfavorable during the whole period. 
The fact that the animals tend to leave clear water only dur- 
ing a short time after being placed in it is undoubtedly due 
