NATURAL HISTORY OF ONCHIDIUM 451 
commence a baisser," and that they again seek shelter in these 
cavities Avith the return of high water. But the pecuUarly 
significant fact that each Onchidium returns to a particular 
crevice, Joj^eux-Laffuie did not discover (if indeed it occurs in 
his species). He pointed out that in 0. celticum, as in our 
form, the attachment of the foot to the substratum is feeble, so 
that the snail if exposed under water would find it impossible to 
retain its footing. He states also that ^'Le moment ou elle sort 
des fentes des rochers et celui ou elle s'y refugie sont tres variables, 
suivant la temperature," this species rarely emerging in winter; 
moreover, ''par un temps convert et humide, elle se promenent 
beaucoup plus longtemps que par un temps clair et sec." For 
0. fioridanum we can agree that temperature is probably a factor 
in this matter, but the degree of atmospheric humidity has 
seemed quite insignificant. According to Bretnall ('19), two 
species of Onchidium observed by him in Australia did not show 
the possession of 'homing habits.' No details are given. 
Further data regarding the curious homing behavior of Onchid- 
ium, together with such analysis of the situation as may be 
attempted on the basis of our inquiries, are best deferred until 
something has been said concerning the sensory reactions of 
Onchidium. Some additional features of the creature's natural 
histoiy must first be presented. 
Locomotion 
The mechanism of the pedal creeping of Onchidium has been 
noted by several observers (cf. Parker, '11; Olmsted, '17a). As 
in the case of many other pulmonates, progression is accom- 
plished by means of pedal waves originating at the posterior end 
of the foot and coursing anteriorly (direct waves). Ordinarily 
but one of these waves, extending across the whole width of the 
foot (monotaxic), is present on the foot at a time. Another 
wave is initiated just as its predecessor reaches the anterior end 
of the foot. Sometimes two waves are visible at one time, the 
second having been commenced just before the disappearance 
of the first. A wave causes the posterior extremity of the foot 
