462 LESLIE B. AREY AND W. J. CROZIER 
tentacle and oral lappet — and the corresponding behavior of the 
analogous organs of a nudibranch, such as Chromodoris (Crozier 
and Arey, '19b), should be noted here. 
The high tactile irritability of the oral lappets was noted by 
Joyeux-Laffuie ('82, p. 311), who described also the richness of 
their epithelial innervation and the manner in which, as we 
have already described, these organs are constantly engaged in 
'feeling over' the substratum during creeping. 
Onchidia placed in a trough through which a gentle current of 
sea-water is maintained soon become oriented by the current 
and creep with it. It is not altogether certain to what extent 
such orientation may be a purely passive one, for to an under- 
water surface the foot of Onchidium is not very firmly attached, 
so that, as the anterior end of the foot is sometimes lifted, this 
part of the body may be mechanically swung around with the 
current. 
A location affording on a calm, sunny day several hundred 
Onchidia may be searched in vain for a single one if a strong 
wind be blowing from such a direction as to impinge upon this 
particular stretch of shore, while at other places, sheltered from 
the wind, the usual complement of feeding snails is seen at low 
water. With the idea that perhaps the explanation might here 
be found for the non-emergence of Onchidium during stormy 
periods, individuals were taken from the laboratory stock and 
allowed to creep upon a horizontal slab of stone freely exposed 
to the wind. They became promptly oriented so as to head 
away from the wind. The tentacles were sharply retracted 
when first struck by the breeze, then subsequently slightly 
extended. WTien the tentacles were removed by a quick snip 
with scissors, these animals were not longer oriented by the 
wind. If but one tentacle was removed, an Onchidium was 
found to go through a sort of 'circus movement,' tending strongly 
to bend toward the unstimulated (non-tentacled) side. The 
tentacles seem therefore to serve as anemotropic receptors. In 
agreement with this a number of rocks containing Onchidium 
colonies were found to have their upper surfaces and windward 
sides free of exposed Onchidia, whereas the protected leeward 
