ZOOLOGY: AREY AND CROZIER 
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any tendency to follow its own slime-track or that of another individual; nor 
does it 'favor' a wet or a dry surface, a rough or a smooth one, in any detecta- 
ble way (as, for example, an earthworm does). 
It may be considered that some form of contact irritability, perhaps resi- 
dent in the oral lapets, is partially responsible for this behavior. Experi- 
mental tests of this point, which are difficult to secure, are not yet complete. 
If the path ordinarily followed in the immediate vicinity of the nest be scraped 
bare, or the rock at the very entrance to the nest be chiseled away, the On- 
chidia endeavoring to return there collect at the edge of the cleaned area and 
then wander about in its neighborhood until they are covered by the tide 
(and washed off the rock). 
There are additional features, of the first importance: An Onchidium found 
returning to its nest may be carried 50 cm. away and placed on rock above 
high water level, where these animals never go naturally, and in at least half 
the trials the snail succeeds in getting back to its nest. If a nest to which the 
inmates have returned be broken open and the animals placed on the rock at 
distances up to 50 cm. from the former entrance, they have no serious dif- 
ficulty in making their way to the location of the old home. 3 Onchidia from 
neighboring colonies, or from laboratory stock, make no attempt to enter 
such a nest. 
We are therefore forced to the provisional opinion that an Onchidium re- 
turns to its particular nest by virtue of some internal condition, simulating 
memory of the position of this nest in terms of its surroundings, but inde- 
pendently of the guidance which may be afforded by mechanically directive 
features of the environment. 
To the extent that the homing movements of Onchidium may be proved to 
involve associative memory, this snail may be placed in a series comprising 
such types as Chiton, Fisurella, Onchidium, and Octopus, all four of which, 
in a sense, exhibit homing behavior, but of increasing degrees of precision and 
complexity in the order of the arrangement here given. The further study of 
Onchidium, both for itself and in relation to these other mollusks, should give 
rise to some valuable conclusions. 
Summary. — Onchidium floridanum lives during high tide in 'nests,' cavi- 
ties in the rock containing a number of individuals, from which the mollusks 
at low water emerge to feed upon exposed shore surfaces. The individuals 
emanating from any one nest return simultaneously to that nest before the 
tide rises again, and during this return give evidence of homing behavior, 
which seems to depend mainly upon internal conditions akin to the remem- 
brance of specific localities. 
1 Contributions from the Bermuda Biological Station for Research, No. 95. 
2 A more complete account of these observations, with supplementary material, will be 
found in our paper, 'On the behavior of Onchidium,' shortly appearing elsewhere. 
3 When a nest has two openings, the removed inmates, or some of them, may turn di- 
rectly to the second opening and enter there, even if they have in the first instance employed 
the other entrance. 
