476 LESLIE B. AREY AND W. J. CROZIER 
The resistance of Onchidium to high temperatures is decidedly 
greater than that of Chiton (Arey and Grozier, '19), which is 
worth pointing out because these animals live almost side by 
side, the Chitons probably being exposed to higher temperatures, 
and for longer periods, than Onchidium. The blue-black hue of 
most of the Onchidia may lead to the absorption of heat energy, 
however, during the intervals of creeping in the open. An 
adaptive basis might therefore conceivably be found for the 
relatively high heat resistance of the dark Onchidia used in these 
tests, if it should be found that these dark forms show a heat 
resistance superior to that of the pale variety obtained at nearby 
stations. 
Jj-. Chemical excitation 
Onchidium does not react in any definitely detectable way 
when a small volume (0.5 cc.) of rain-water is applied to any 
part of its surface. Sea-water itseK, so applied, may provide a 
mechanical stimulus unless the pipet current be very gentle; 
this effect, however, may readily be eliminated. Sugars (mal- 
tose, lactose, sucrose) in 1 M solution in rain-water were not 
found to induce responses. Sea-water concentrated to one-half 
its volume by evaporation and reaerated by bubbling air through 
it called forth contractile movements when applied to the head 
and lips; sea-water whose osmotic pressure had been increased 
by the addition of 0.5 M glycerin similarly induced slight reac- 
tions when applied to the lip region. 
The osmotic disturbances which may induce responses about 
the mouth are much less effective as activatmg agents for Onchid- 
ium than are various chemicals, including alkaloids, anaes- 
thetics, alcohols, acids, alkalies, salts, and such irritants as 
H2O2 and various • essential oils. The whole surface of Onchid- 
ium is open to sensory activation through the agency of dilute 
solutions, such as those found effective for numerous other inver- 
tebrates (cf. Arey and Crozier, '19). 
Small volumes of sea-water which had been shaken with ether, 
chloroform, carbon bisulphide, aniline oil, oils of thyme, ori- 
ganum, juniper, bergamot, pennyroyal, cloves, or cassia, were 
