The Heliotropism of Onchidium: a Problem in the Analysis of Animal Conduct

Onchidiumfloridanum is a small naked pulmonate which lives in holes and crevices of the shore between tides. A description of the bionomics of this mollusk we shall present more fully in another connection, 1 omitting here those aspects not bearing directly upon the most curious puzzle propounded by its behavior, which in brief is this: under natural conditions the movements of Onchidium take place in the light, but without reference to photic control; yet When the animal is tested apart from its natural surroundings it is found always to be negatively phototropic. In view of this apparent incompatibility, in reality very deep seated, one is led to inquire: Is the behavior of Onchidium an instance where "unnatural conditions" imposed in phototropism experiments vitiate attempted analytic explanation of normal acti~ties and more especially what, fundamentally, is the sense of the state of affairs we have outlined, in terms of adaptation?