NATURAL HISTORY OF ONCHIDIUM 463 
faces of these stones bore many feeding individuals. In one 
case three of these had their tentacles removed, and when placed 
in exposed situations crept into the breeze without hesitation. 
It is probable, however, that a still stronger wind would orient 
detentacled Onchidia. (No tests were made of the orientation 
of detentacled snails in a water current.) 
It should be mentioned here that in the tips of the tentacles 
there are found considerable numbers "of nerve cells; the possi- 
bility is suggestive that they may be important of anemotropic 
sensitivity. 
With 0. celticum, Joyeux-Laffuie noted that certain stretches 
of shore might harbor many hundred Onchidia, whereas nearby 
stations exhibited few or none; he did not believe that the char- 
acter of the food supply was instrumental in determining the 
erratic character of this distribution, but gave no evidence bear- 
ing upon the real explanation. Similar facts are quite evident 
at Bermuda, and we regard it as clear from our own studies that 
the degree of exposure to wind, and possibly surf, is a prepon- 
derating factor in the matter. Protection from the force of 
wind and surf is essential, and at stations notably deficient in 
this particular no Onchidia are found. 
Onchidia placed at the bottom of a battery jar soon climb its 
side, and generally do so in a straight line perpendicular to the 
ground. If the dish be covered they will accumulate at the 
top, some remaining on the vertical surface, others upside down 
on the glass cover; particularly during the first hours of such a 
test it is rare to see even one individual moving downw^ard again, 
and the whole group will usually stay at the top for days. In 
the absence of a cover, however, the same animals readily creep 
over the edge and on, downward, to the table. Onchidia placed 
on a glass plate seemed on the whole to be negatively geotropic, 
since in many instances appropriate reversals in the direction of 
creeping could be induced by turning the plate; such results 
were not always forthcoming, however. The natural situations 
of the Onchidium nests involve normally some degree of upward 
or downward creeping, since not infrequently the opening of a 
nest will be on an almost vertical surface. But aside from the 
