NATURAL HISTORY OF ONCHIDIUM 483 
by Joyeux-Laffiiie ('82), namely, that the tentacular eyes enabled 
the molliisk to distinguish between the darkened interior of its 
shelter and the illuminated outside feeding ground, thus guiding 
its emergence.^ The difficulties facing such interpretation are: 
1) the fact that the tentacles of 0. floridanum, although there 
are tentacular eyes, seem non-photosensitive; 2) the fact that 
the creature is never positively heliotropic and, 3) the fact that 
emergence does not always occur when the tide is out during 
daylight hours. Good instances of the sort last mentioned 
were available in midsummer, when for several days at each 
lunar interval both tides were seen to expose the beach zone 
while the illumination was still quite good. At these times the 
snails were always found to emerge during but one tidal ebb, 
never during both. Once in each twenty-four hours is the 
maximum frequency of emergence. Even in the absence of 
conditions imposed by winds and currents impeding the escape 
of tidal water from the sounds, however, this rhj^thm does not 
represent the minimum frequency of emergence. For at neap 
tides the Onchidium nests highest above water may fail for some 
twenty-four hours to be submerged at flood tide, and in that case 
the Onchidia there located do not emerge until after their nest has 
been submerged. Moreover, in winter both morning and even- 
ing tides may fail for a day or so to occur during daylight hours; 
in this event, so far as we have studied the point (Dec, Feb., 
1918), the Onchidia do not emerge at all until one tide occurs 
while the sun is up. 
The diurnal rhythm thus clearly established 4n defiance of 
the snail's heliotropism completely disappears in the laboratory. 
Fair-sized slabs of stone, a foot or so in breadth, were placed in 
aquaria containing freshly gathered groups of Onchidium. The 
animals always collected after a short time on the shaded, under 
side of such a slab, whether under water or in air. In the dark 
(artificial darkness or at night), they crept actively over the 
surface of their stones, feeding; the admission of light quickly 
caused typical photonegative retreat to the dark, under surface 
* The fact that emergence occurs only during the daytime was not known to 
Joyeux-Laffuie. 
