990 THE AMERICAN NATURALIST. [VOL XXXV. 
coiled that the covering is almost spherical, or the body may 
simply be bent back upon itself at one or two points, thus 
giving the covering.an elongated form. The worm often 
remains in this covering for days or even for weeks, but may 
leave it at any time. That the animal is not incapable of 
making its way out is shown by the fact that violent stimu- 
lation will usually cause movements of sufficient force to 
rupture the wall of the covering and free the enclosed worm. 
The most interesting fact, and one which, as I believe, throws 
some light on the nature of this phenomenon, is that even 
after repeated stimulation or after rupture of the wall with 
needles, the animal in many cases exhibits an apparent reluc- 
tance to leave the covering. The head is protruded, makes 
the usual searching movements, and then is withdrawn again, 
and the worm continues its slow movements within the cavity - 
of its case. 
I believe that the enclosure of the animal within this secre- 
tion, and its apparent reluctance to leave its covering, are the 
result of the strong positive thigmotaxis which other actions 
show to be present. In a jar of clear water the slight accumu- 
lations of slime, which are the result either of irritation by 
strong stimuli or have been formed by the continuous secretion 
of slime at some particular spot where the animal has rested 
for a time, serve in some degree as a contact stimulus, and the 
worm tends to remain in or about them. Continued secretion 
leads to the formation of a more or less definite coat of slime, 
within which a cavity is formed by the continuous crawling of 
the worm. Various parts of the coiled and twisted body stimu- 
late the sensitive head, giving the sensation of contact, and the 
head may often be seen to force its way in between two moving 
coils of the body, the whole body following. Thus the animal 
is constantly forming new figures and arranging its body in 
complex coils within the slime. From time to time, if the 
animal is well fed, the faeces and intestinal mucus are added 
to the wall, which thus becomes whitish and more opaque. 
Within the covering, then, the impulse which leads the animal 
to seek contact with surfaces is satisfied, and a strong stimulus 
is necessary in order to induce emergence. This latter fact 
