NATURAL HISTORY OF ONCHIDIUM 
467 
mantle-fold being promptly depressed; at the posterior end, 
when shaded locally (from behind), the posterior part of the 
body is likewise contracted and depressed, but the contraction 
of this portion of the body is not so pronounced as when, 
in the case of anterior shading or of the shading of the whole 
body, the anterior part of the body seems to be drawn backward. 
No reaction follows the shading of the tentacles alone, but if 
the anterior edge only of the mantle be shaded, a normal reac- 
tion follows. Onchidia from which the tentacles have been 
TABLE 1 
Showing the course of exhaustion of the response to shading in three individuals 
{Onchidium) in bright sunlight; shaded at three-minute intervals 
NUMBER OF 
RESPONSE 
Animal 1 
Animal 2 
Animal 3 
1 
Complete reaction 
Complete reaction 
Complete reaction 
2 
Complete reaction 
Only tentacles re- 
tracted (slightly) 
Weak total response 
3 
Reaction mostly from 
Just perceptible re- 
Good complete reac- 
the tentacles 
sponse of tentacles 
tion 
4 
Tentacles retract 
One tentacle partial- 
Tentacles only, re- 
slightly 
ly contracted 
tracted 
5 
Tentacles retract just 
One tentacle slightly 
Tentacles only, re- 
perceptibly 
bent away 
tracted 
6 
One tentacle caused 
to bend to one side 
7 
One tentacle caused 
to bend to one side 
amputated are normally responsive to shading, even within five 
minutes subsequent to the removal of the tentacles. 
Reactivity to shading is quickly exhausted by rapidly repeated 
activation. In bright sunlight, one or two 'complete' reactions 
are all that are usually obtainable when the shadings succeed 
one another at intervals of 3 mm. (cf. table 1). Successive shad- 
ings evoke responses gradually less complete. 
When the receptivity of the anterior end for shading stimula- 
tion is completely exhausted, this part is nevertheless fully 
responsive to delicate tactile activation. Mam^ tests were 
