NATURAL HISTORY OF ONCHIDIUM 479 
as in the case of resting individuals. These features may play a 
part in directing the movements of Onchidium in nature. 
There is some evidence for the \'iew that receptors for general 
'irritating' activation are distinct from chemoreceptors proper. 
Many substances which provide powerful sensory excitants for 
Onchidium do not induce discharge of the repugnatorial glands. 
Methjd or ethjd alcohol, however, at 5 M concentration in rain- 
water, do lead to such discharge; whereas pure amyl alcohol, 
directly applied, does not, although the general responses of the 
animal are in the case of the amyl alcohol much the more vigor- 
ous. Chloretone also, and urea, in relatively concentrated solu- 
tions, do not lead to discharge of the poison glands, although 
they do induce powerful reactions on the part of the animals as 
a whole. 
An effort to effect physiological separation of tactile- and 
chemo-reception was without decisive result. 
It may be noted that, according to Joyeux-Laffuie ('82, p. 
238), 0. celticum exhibits in its feeding a certain amount of pref- 
erential selection of Ulva from among a diversity of algae. He 
regarded this fact as evidence of a certain degree of 'gustatory' 
discrimination, associated with the mouth parts, but pointed 
out in addition that the oral lappets were employed in feeling 
over bits of algae before their being submitted to the radula. 
This latter observation we can confirm for 0. floridanum, but 
we have seen no evidence of selective activity respecting food, 
perhaps because the thin algal carpeting of the rock in Onchid- 
ium habitats presents so uniform a field. Some of the older 
writers have referred to the 'eating' of silt by Onchidium; we 
have already pointed out (Crozier and Arey, '19 a) that shore-line 
silt is ingested, but in a purely fortuitous manner, through its 
adherence to the seaweeds, 
ON THE ANALYSIS OF THE HOMING BEHAVIOR 
An attempt to interpret or to reconstruct the natural activi- 
ties of Onchidium on the basis of analytical study of its modes of 
response when removed from its habitual environment is con- 
fronted at once by some profound inconsistencies and by the 
