NATURAL HISTORY OF ONCHIDIUM 493 
For reasons already amply set forth, we must reject the notion 
that the movements of Onchidium involve, or depend upon, 
any 'reversal' of phototropism. From the standpoint of adap- 
tation, the heliotropic mechanism must be regarded as a most 
interesting example of a perfectly definite functional character- 
istic which proceeds automatically from the given physicochem- 
ical composition of the organism (cf. Loeb, '16), without refer- 
ence to adaptive requirements (cf. Arey and Crozier, '18; Crozier 
and Arey, '19 c). Since the young of Onchidium (developing 
within the nest) emerge from the egg capsule with the form of 
the adult, and not as veUgers (Joyeux-Laffuie, '82), and since 
we have found very tiny individuals (2 mm. long) emerging from 
nests with adults, it cannot be said that perhaps at an early 
stage these anunals are normally photonegative, bj^ this means 
first becoming established in their definitive nest. 
4. A number of instances are on record of the preservation in 
the rhythmic activities of anunals of some diurnal or tidal 
rhythmicity inherent in the environment (Wilson, '00; Gamble 
and Keeble, '00; Schleip, '10; Keeble, '10; Esterly, '17; Gary, in 
Dahlgren, '16, reprint p. 11, etc.). Unfortunately, a number of 
such reports, especiall}^ those concerning the persistence of 
environmental rhythms in actinians, have proved to be the 
result of erroneous observation (Parker, '19). We were inter- 
ested to discover if, in a form like Onchidium exhibiting such 
complex responses, there would be found any persistence of 
either tidal or nycthemeral rhythms of activity and repose, in 
the absence of the rhythmic excitations normally associated. It 
can be said with confidence that no rhythms of this character 
are maintained by Onchidium when removed to the laboratory. 
5. For some time it has been known that the limpets and their 
allies inhabiting the tidal zone may at times wander for some 
little distance from, and subsequently return to, the 'scar' 
indicating their definite 'home.' The Uterature of this subject 
has been reviewed in an interesting way by Pieron ('09 c). A 
certain complication enters here, for some limpets creep forth 
from their scar when covered by the sea, others only when left 
bare by the tide. Pieron (loc. cit.) has given a plausible account 
