NATURAL HISTORY OF ONCHIDIUM 461 
The ventral surface of the projecting portion of the mantle 
when stimulated undergoes reactions of a sort similar to the 
preceding at the anterior or the posterior extremity. At the 
sides of the body, the mantle locally bends ventralward toward 
the substratum when its lower surface is activated. If the 
periphery of the foot is touched, the substance of the foot is 
puckered away from the source of stimulation, and the mantle 
is depressed at this point. The ends of the foot are more sensi- 
tive than its lateral edges. Stimulation of the end of the foot 
causes the animal as a whole to contract, arching its dorsum. 
The ventral surface of the foot, which may be studied by allow- 
ing the Onchidium to creep over a gap between two glass plates, 
reacts negatively to the tactile activation of a blunt-pointed 
instrument, and the lateral margins of the foot on both sides at 
the level of stimulation contract locally toward the median line. 
The tentacles and the oral lappets are the parts most sensitive 
to touch. A tentacle stimulated at its tip or at a point along 
its stalk is quickly rolled inward, glove-finger fashion, like the 
tentacle of a snail; it is then re-extended more slowlj^ The 
response is unilateral. Unsymmetrical tactile excitation of the 
tentacles may be used to direct the path of locomotion, as was 
attested by the fact that an Onchidium moving away from a 
source of light could be made to alter its direction by repeatedly 
touching one tentacle; the animal turns away from the stimu- 
lated side. 
Activation of the anterior portion of the mantle-fold at one 
side results in the contraction of the tentacle, of the oral lappet, 
and of the head as a whole on the homolateral side; the mantle- 
fold is itseK at the same time locally depressed. More intense 
stimulation or a light touch repeated four to six times leads to a 
similar response from the opposite side of the head as well. 
If a tentacle be very Ughtly touched, it alone responds; if 
somewhat more strongly stimulated, the homolateral oral lappet 
is also involved in the reaction. An oral lappet, however, will 
respond repeatedly without the homolateral tentacle being impli- 
cated. The decided parallelism between these peculiarities — 
homolaterality of response and irreciprocal conduction between 
