460 LESLIE B. AEEY AND W. J. CROZIER 
stimulation of the tentacles, occupies a well-defined place in the 
bionomics of Onchidium. Geotropism is not well defined and 
reactions to vibratory stimuli are but poorly represented. 
WTien the back of an Onchidium is momentarily touched, a 
slight local depression is formed. The dorsum is not very active 
to a single light touch. If the dorsum be 'scratched,' how- 
ever, or stroked several times in succession with a blunt point, 
the particular area affected becomes to some extent contracted; 
but the most obvious response is from the margin of the mantle — 
the mantle-fold is erected, forming a saucer-like rim about the 
body, so that the now erected repugnatorial glands come to 
point in a general way toward the spot irritated. 
The marginal zone of the mantle is much more sensitive to 
touch than is the back of the animal. If the anterior end of a 
creeping individual be lightly touched, the tentacles and oral 
lappets instantly retract, the mantle-fold is depressed to the 
substratum, the back of the animal becomes strongl}' arched, 
and locomotion ceases. Three or four gentle stimulations in 
succession are required to induce the completion of this form of 
response, but a single touch is sufficient to bring about the expres- 
sion of its initial phases. Almost immediately after the stimu- 
lation has ceased, the head and tentacles are protruded from 
beneath the mantle and locomotion is resumed. If the animal 
is, to begin with, not creeping, but quietly attached in its char- 
acteristic attitude (the head and tentacles being withdrawn, the 
body then appearing oval or circular in outline), a touch causes 
the edge of the mantle to be retracted and the back more decid- 
edly arched. Stimulation of the posterior end of the mantle of a 
resting Onchidium causes the part affected to be drawn forward 
and curled under the body. In a creeping individual, if the 
posterior end be touched several times in succession, the whole 
posterior third of the body is contracted, so that in outline, 
seen from above, it is pear-shaped; but the locomotion does not 
cease, and the animal continues to creep with its posterior part 
contracted in this way for several minutes. The peripheral 
edge of the mantle is quite sensitive to touch, reacting by local 
retraction. 
