NATURAL HISTORY OF ONCHIDIUM 477 
allowed to flow over a portion of the surface of Onchidium; the 
reactions were in every case very vigorous, no matter what part 
was immediately concerned in the activation — the foot, for 
example, being found extremely sensitive. 
Low concentrations of electrolytes, however, are more effec- 
tive on the dorsal mantle and in the mouth region than on the 
foot. The limiting concentrations beyond which the further 
dilution of some representative materials no longer constitutes 
sensory stimulants under the conditions of these tests — in a 
general way similar to those found for other animals — are: 
HCl — The foot ceases to react when the solution is more dilute than 
M/300; the remainder of the body surface responds to dilutions as 
great as M/500. 
KOH — The limiting concentrations are about the same as for HCl, 
but somewhat higher, 
KCl— For the foot, M/3; for the rest of the body, roughly M/5 or 
M/6. ^ 
Picric acid — At M/50 a weak response is still obtainable from the 
foot; the limiting dilution for the general body surface is about M/3000; 
at M/2000 the stimulation induced is still quite decided. 
The animals were tested in air. The solutions were made up in 
rain-water; with picric acid, solution in sea-water gave results 
identical with those already tabulated. 
It was attempted, for comparison with conditions in other 
forms, to establish the relative stunulating efficiency of some of 
the commoner ions. Solutions of the neutral salts of the alka- 
lies, 5/8 M in rain-water, induced responses from all parts of 
Onchidium. MgCL, even in 1 M solution, gave weak reactions 
only, and none whatever upon the foot. 
NaCl and LiCl led to the explosive discharge of the repugna- 
torial glands, whereas KCl and NH4CI as a rule did not. When 
local applications of the two latter salts were followed by a gland 
discharge, it could be seen that the discharge was a secondary 
phenomenon, due to the general squeezing of the glands through 
the muscular contraction of the mantle, rather than to the 
(normal) contraction of the investments of the glands them- 
selves, the latter holding with NaCl and LiCl. NaCl in small 
quantities led, on the whole, to more violent gland discharge 
