NATURAL HISTORY OF ONCHIDIUM 475 
The lowest of the high temperatures which leads to death 
rapidlj^ (i.e., within one minute) is very nearly 48°, although 
47° is withstood for ten to fifteen minutes. 
Local appUcation of 'heat' or 'cold'— the tests being made by 
bringing a warmed or cooled glass rod into close proximity with 
a part of the snail, in air, or, since it was possible by careful 
manipulation to avoid tactile comphcations, bringing the glass 
rod directly into contact with the skin — provided no evidence of 
dehcate thermal sensitivity. A rod cooled approximately to 
0° C. called forth no detectable response whatever. A rod 
heated to 60°, or even 50°, did, however, cause prompt responses 
from all parts. The type of reaction was for each part of the 
body the same as in the case of touch, but more vigorously car- 
ried out. A Ught touch leadmg to no response at all calls forth 
a powerful reaction when administered with a warmed glass 
rod. The mimimal temperature effective in this way is about 
45°. 
In the case of an Onchidium detached from the substratum 
and resting on its back, the lower surface of the mantle may be 
touched repeatedly on one spot until the maximal amount of 
rolUng-up occurs; there is then no further response obtainable to 
repeated mechanical stimulation, but a rod warmed to 45° (in 
reahty, perhaps somewhat cooler at the moment of application) 
induces a sharp local contraction and drawing away of the 
mantle. Similarly, the back of an Onchidium may be repeatedly 
stimulated by pressing on it with a blunt point, by prodding or 
stroking it, until a response fails to be ehcited; but a warmed 
rod, however hghtly applied, calls forth a deep retractive puck- 
ering of the mantle over a relatively wide area. 
The findings in these somewhat tedious experiments indicate 
the presence of a 'heat' sense physiologically distinct from that 
involved in touch, although (perhaps owing to the method of 
experimentation) the relative sensitivity of the various parts of 
the body seems the same as that already described for mechan- 
ical excitation. Sensitivity to 'cold,' on the other hand, is but 
poorly defined, if indeed it can be said to be present at all. 
THE JOURNAL OF EXPERIMENTAL ZOOLOGT, VOL. 32, NO. 3 
