NATURAL HISTORY OF ONCHIDIUM 453 
rupted if the dorsum of the animal is pinched or prodded, since 
the periphery of the mantle with its repugnatorial glands is then 
elevated and caused to bend toward the focus of irritation. 
The animal nevertheless continues to adhere with some firmness to 
the rock. The major share of this adhesion is apparently due to 
slime, as in other pulmonates. Onchidium can maintain a position 
upside down on a glass surface when less than one-third of the 
foot is in contact; it can creep over a crack between two glass 
plates; it creeps undisturbed over a hole in a glass plate, the sub- 
stance of the foot being pressed firmly into the vacant space. 
The adhesion of the foot is due, then, in the first place, to the 
close contact brought about between the foot and the rock or 
other surface, and, secondly, to the slime which the foot secretes; 
possibly local suction on the part of minute areas of the foot is 
also concerned, as m Chiton, although this cannot easily be 
tested. Usually, if not invariably, the attachment of the foot 
can begin at its anterior end only, although the animal may for 
some minutes maintain its weight by means of adhesion through 
the posterior third of the foot surface alone. 
When rolled over on its back, an Onchidium usually begins 
soon to right itself. Some individuals, in the laboratory, are 
quite inactive, however, and will not begin to right themselves 
for a long time. If turned over while under water, an Onchidium 
seemingly active in other ways may remain on its back for sev- 
eral hours, much longer than when in air. In righting, pedal 
waves of considerable amplitude are formed and continue to pass 
over the foot until righting is accomplished. When first dis- 
placed, the animal tends to curl up like a Chiton, but soon 
stretches out again. The anterior end is twisted and contracted 
on one side until the anterior portion of the foot can be attached. 
Righting then proceeds as the attachment of the pedal surface 
progresses toward the posterior end, so that when half attached 
the body of the Onchidium is twisted through an angle of 180°. 
Under ordinary conditions an Onchidium will right itself in 
15 to 30 seconds (mean, 25 seconds). It should be noted that 
the back of an Onchidium is strongly arched, so that when dis- 
placed from the rock, and caused to 'curl up' with its fringe of 
