Behavior of Tubicolous Annelids 165 
selves associated with experiments on fatigue. Some of their 
results are similar to phases of those here under review, and seem 
to confirm the suggestion of fatigue. 
TACTILE EXPERIMENTS 
In the former paper reference was made in several places to 
tactile reactions, but no details of experiments were given. ‘These 
I have subsequently repeated and extended and with such varia- 
tions as to suggest certain inferences and conclusions not previously 
considered. 
As might naturally be expected in creatures whose sensory 
powers are as delicately adjusted as the reactions to light already 
described, the tactile sense is also very acute. And interestingly 
enough it is found most highly developed in the gill filaments, 
the organs concerned with photic sensibility, one and the same 
organ therefore serving the double sensory function of touch 
and photic perception. But this is not peculiar to these organisms. 
Not a few of the lower invertebrates show very similar conditions, 
and suggest the inference that tactile and visual senses are more 
or less intimately correlated. Indeed, but for the conventional 
definitions of these senses, based to a large extent on the highly 
differentiated organs of higher organisms, it might be fairly allow- 
able to regard them as modified expressions of sensory processes 
due to stimuli of fundamentally similar nature. No time can be 
taken in this connection to follow out the suggestion further, but 
it seems well to call attention to the facts, barely hinting at the 
more or less obvious inferences concerned. 
While experimenting with shadow tests it was observed that 
now and then a specimen was found whose reactions were markedly 
inferior, or lacking entirely at times. Something of this will be 
noticed in the tables. In order to determine that in such cases 
the behavior was not due to some abnormal or pathologic condi- 
tion the specimens were subjected to tactile tests, and in almost 
every case were found to respond as promptly to such treatment 
as any of the others. And in not a few such cases it was found that 
a few tactile tests sufficed to awaken, as it were, the dormant pho- 
