474 LESLIE B. AREY AND W. J. CROZIER 
none whatever, could be secured; this was true even when the 
hght falhng on the bkie filter was made very weak, while that 
admitted to the red filter was much stronger. 
3. Thermal excitation 
Under water Onchidium remains 'normal' in its behavior, so 
far as can be told, at temperatures between 17° and 36°C. If 
placed in sea-water cooled to 5°, the animal becomes instantly 
immobile, and does not respond to touch. After ten to fifteen 
minutes at this temperature, Onchidium quickly recovers if 
transferred to sea-water at the normal temperature (26° to 27°) ; 
in the early stages of this recovery no reactions of any sort could 
be ehcited by tactile agents, but good reactions, involving con- 
traction and curling up of the whole body, were elicited by the 
local application of small volumes of dilute HNO3 (N/200 ±). 
Sea-water cooled to 10° has practically the same effect. At 15°, 
an Onchidium suddenly placed in water of this temperature 
usually exhibits a few sluggish contractions of the foot muscula- 
ture; if the animal is expanded, it contracts a little; if con- 
tracted, it relaxes somewhat and then remains quiet. Tactile 
irritabiUty is present, but the responses are of shght amplitude. 
Above 17°, probably, certainly above 20°, and until a tempera- 
ture of 35° to 36° is used, no changes in the behavior of Onchid- 
ium indicative of sensory activation by heat are obtainable. 
At 35° to 38°, Onchidia transferred to sea-water of these tem- 
peratures quickly become motionless, in the expanded state, 
except that if placed ventral surface uppermost there seems to 
be a shght but detectable increase in the peristaltic activity of 
the foot. They may make attempts to right themselves, but 
the bending movements involved in this process do not continue 
after two to three minutes. 
With temperatures up to 45°, the result of immersing the 
animal suddenly is to call forth rather violent general contrac- 
tions, more powerful the higher the temperature, but lasting 
less than one minute at 45°. Onchidium endures exposure to 
water of the latter temperature for about forty-five minutes; 
even after thirty minutes' exposure, it still reacts to touch and 
to stimulation by weak acid (HNO3) solutions. 
