386 
PACIFIC SCIENCE, Vol. IX, October, 1955 
probability, could not be held entirely re- 
sponsible for this phenomenon. 
However as blinded H. pacifica reacted to 
Physalia solutions almost immediately, the 
presence of some selective chemical sense is 
demonstrated and, in light of similar research 
among other arthropods, attention was at 
once centered on antennae, antennules and 
mouth parts. 
The removal of the antennae of blinded 
H. pacifica and the subsequent increase in 
reaction time (3-5 minutes) indicates that the 
chemical sense is in part located on these 
structures. Likewise, the removal of the an- 
tennules of blinded H. pacifica and the sub- 
sequent increase in reaction time (3-5 min- 
utes) also indicates that the chemical sense 
is in part located on these structures. Inas- 
much as the increased reaction time following 
the removal of the antennae so nearly approx- 
imates the increased reaction time following 
the removal of the antennules, one can con- 
clude that these two structures are equally 
important in the degree of selectivity of the 
chemical sense. 
To determine whether the chemoreceptors 
were limited to the antennae and antennules 
in blinded H. pacifica , both structures were 
removed. The increased reaction time (15-21 
minutes) following the removal of both an- 
tennae and antennules indicates that probably 
there are chemoreceptors present on the still 
intact mouth parts, although they are much 
less effective than those of the antennae and 
antennules. 
The chemoreceptors located on antennae 
and antennules of H. pacifica are therefore, 
responsible for the detection of Physalia dur- 
ing the high surf; the sense of sight being 
responsible primarily for actual location and 
subsequent seizure. This same chemical sense 
must also be responsible for detection, during 
calm seas, of shrimp and Velella which H. 
pacifica quite readily seizes once its digestive 
tract is empty and its preferred food scarce. 
Neither the presence of shredded Physalia 
in the digestive tracts, nor the absence of 
minute organisms enmeshed in the setae of 
the antennae and antennules precludes the 
possibility of unselective feeding by H. paci- 
fica, , especially during the weeks of calm which 
often follow periods of heavy surf. Inasmuch 
as during periods of calm sand grains and 
mucus but no microorganisms have been 
found in the digestive tract of H. pacifica the 
question of unselective feeding remains un- 
answered. 
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