NATURAL HISTORY OF ONCHIDIUM 487 
ing feeding are known in such animals as the Porthesia cater- 
pillars of Loeb ('18, p. 116), and the Planarians studied by Olm- 
sted ('17 b). This hypothesis readily accounts for the fact that 
the period of feeding lasts very nearly the same length of time 
in all the members of a group. 
The sensory impulses thus conceived to be released from 
central inhibition through the results of feeding are regarded as 
originating in the oral lappets. These well-developed 'cephalic 
tentacles' are constantly in touch with the algal carpet of the 
stone. If they are cut off, the Onchidium is 'lost,' unable to 
return to its home. The removal of the dorsal tentacles, some- 
times regarded as the seat of 'smell' in snails, has no such effect. 
These impulses must be regarded as possessing the character- 
istics of 'contact odors' (meaning thereby that perhaps both con- 
tact and 'olfactory' stimuli of a certain kind must be received 
simultaneousl}^) . The reason for this assumption is twofold: 
in the first place, an Onchidium beneath which there is slipped a 
glass plate is left thereby at the mercy of its heliotropism; sec- 
ondly, an Onchidium will 'home' from points which it has not 
previously visited; therefore, the aereal dissemination of some 
guiding substance must be presumed. The olfactory com- 
ponent of such a complex must be regarded as more important 
than the tactile, for the rock surface above high-water mark is 
not covered by algae as is the surface natural for Onchidium, 
nevertheless the snails will 'home' from points on the former 
surface, although in the ordmary course of events they never go 
above high-water mark. They will not home when put under 
sea-water, even if quite near their nest. 
The substance providing a tropistic guide for a fed Onchidium 
must be granted some highly specific quality. In view of 
Bethe's findings for ant colonies ('98), such a supposition need 
not be thought preposterous. Moreover, it is supported by some 
striking results in our experiments on homing. In a number of 
trials an Onchidium from one community was so placed that it 
was forced to creep across the sunken gully leading to the opening 
of a strange nest. Sometimes such a snail was found to follow 
the new 'trail,' after a certain amount of preliminary turning 
