NATURAL HISTORY OF ONCHIDIUM 455 
in 200 cc. of rain-water would live in some cases for 4 hours (at 
27°C.). When first immersed the animal partly rolls up, lying 
on its side or back, and it so remains until dead. If stimulated 
by touching, it momentarily writhes about, then returns to 
quietness. A fair degree of sensitivity is retained for at least 3 
hours in rain-water. The intertidal Chiton tuberculatus is 
similarly resistant (4 hours), but is in addition protected by the 
completeness with which the girdle can exclude fluids from con- 
tact with the animal's soft tissues (Arey and Crozier, '19). 
The nudibranch Chromodoris is killed by 45 minutes' immersion 
in rain-water (Crozier and Arey, '19b). Resistance to rain-water 
may clearly be of bionomic importance, but that it has originated 
adaptively is in no degree certain; more probably, it depends 
upon the organization which Onchidium has inherited from 
ancestors among the land pulmonates (Perrier, '17). 
Certain phenomena of coloration in 0. floridanum are not 
without interest, especially in view of the fact that there is 
present in this snail an active system of repugnatorial glands. 
We have recently published some discussion of this matter 
(Crozier and Arey, '19 a), and need refer here merely to the 
chief points involved. 
At Bermuda this species exhibits two fairly distinct types of 
pigmentation, a pale type verging upon dull olive-yellow, and, 
much more abundant, a type of dark blue-black appearance. 
These two kinds of coloration are found in other species of 
Onchidium (cf. Eliot, '99; Dall and Simpson, '01) and in the 
related genus Onchidiella. The lightly pigmented type is often 
concealingly adjusted to its background, but not in every case. 
The color of the dark variety might also be considered a conceal- 
ing match for the mussels^ which cluster about the entrance to 
its nest. But no correlation can be established between specific 
substrata and the pigmentation of the Onchidia which creep over 
them at low tide. In view of this fact and of the further obser- 
vation that the mud-encrusted slime pellicle investing the back 
of Onchidium is commonly removed in mechanical fashion as 
the snail creeps out of its nest, whereas if the pellicle should 
