452 LESLIE B, AREY AND W. J. CROZIER 
to be contracted anteriorly for a distance of 2.5 to 3 mm.; the 
posterior end, moving forward more quickly just before the 
termination of its 'step' than at the beginning, is then attached; 
as the wave begins to move anteriorly, the animal appears to 
brace itself against its point of attachment, the posterior end of 
the foot spreading out against the substratum and seeming as a 
whole even to move backward slightly. The pedal wave — as 
may readily be seen either when Onchidium is creeping on a glass 
plate in air or when placed upon its dorsum so that righting 
movements are begun — represents an area of the foot lifted out 
of contact with the substratum (cf. Olmsted, '17a, p. 235). WTien 
an Onchidium is placed on its back, wave movements appear 
on the foot, and are somewhat magnified as compared with their 
normal size ; inspection of the waves produced in this way shows, 
under the binocular microscope, that the posterior zone of the 
wave concavity is undergoing longitudinal contraction in the 
anterior direction, as it should according to Parker's ('11) view 
of the mechanics of progression in such cases. The pedal wave 
traverses a foot surface of 13 to 14 mm. length in about 5 seconds 
(i.e., at a rate of 16 ± cm. per minute). The rate of progression 
of adult Onchidia, on a smooth surface, is found to be approxi- 
mately 5 cm. per minute, agreeing with that calculated from the 
preceding data regarding the frequency of the waves (20 per 
minute) and the distance observed to be trans versed (2 to 5 
mm.) as the result of a single wave (cf. Peyrega et Vies, '13). 
Like most gastropods, Onchidium creeps only in the anterior 
direction; its creeping appears incapable of reversal. 
Onchidium creeps over substrata of varied texture, such as 
bare rock, corraline algae, or felted enteromorpha, and while 
entering or leaving its 'nest' passes over groups of Modiolus 
presenting a sharply serrated surface. In agreement with the 
possibility of the mollusk's living upon such surfaces, it is found 
that the foot does not serve as a hold-fast through its action as 
sucker. To some extent, as in Chiton (Arey and Crozier, '19 a), 
the mantle serves as a hold-fast. If the animal be touched, or 
shaded (vide infra), the mantle is quickly applied to the rock 
surface, very much as that of Chiton is. This reaction is inter- 
