172 
PACIFIC SCIENCE, Vol. II, July, 1948 
rapidity with which the activity occurred, dem¬ 
onstrates with certainty that visual perception 
was employed. Indeed, an identical response 
could be induced by tossing pebbles near the 
edge of the crevice; the crabs emerged, touched 
the pebbles with the advanced ambulatory legs, 
but failed to pick them up. Response time 
varied from 5 to 30 seconds, and on no occa¬ 
sion was any substance ignored. 
Food lying on the rocks failed to attract the 
crab’s attention, providing accidental contact 
was not made. One crab reached its crevice 
high on the rocks subsequent to the deposition 
of an odorous sardine within 3 inches of the 
refuge opening. A second crab emerged from 
the pool and passed within 6 inches of the 
sardine but ignored it completely. The age of 
the sardine provided no adverse effect upon 
its palatability because it was located and eaten 
during the extensive nocturnal wanderings char¬ 
acteristic of this species. 
The retarded response of P. crassipes toward 
securing food placed in the tide pools suggests 
that this species often depends upon the visual 
stimulation provided by feeding activity of other 
tide-pool residents. To test this suggestion, 
observations were made in a pool which har¬ 
bored no forms other than P. crassipes. Food 
materials were cautiously introduced into the 
pool with the aid of a long pole manipulated 
from behind the blind. The interval which 
elapsed between the introduction of the food 
and the initial response toward obtaining it 
was indicative of the proclivity of the crabs 
toward recognizing its presence. Between 7 
and 8 minutes elapsed before emergence of 
the smaller crabs from crevices to seek the food. 
After discovery, frequent combats occurred. 
The larger crabs appeared soon after the smaller 
ones had located the food. It would seem that 
the movements of the smaller crabs played a 
role in stimulating the larger individuals to 
seek the food material. 
Activity mediated chiefly through chemical 
stimulation was manifest on several occasions. 
During one observation period several fragments 
of abalone (Haliotis rufescens Swainson) were 
carefully placed in one end of a tide pool in 
which crab activity was slight. After 7 minutes 
the food was removed. Apparently no crabs 
had observed its deposition, inasmuch as no 
investigational activity ensued. Shortly there¬ 
after, several small crabs left their crevices and 
began what simulated a random search. Shortly 
after the emergence of small crabs, larger indi¬ 
viduals ventured forth. Within a short time 
most of the crab population of the pool was 
concentrated in the recently baited end. The 
high degree of pugnacity demonstrated by larger 
crabs seemed to indicate that they were perhaps 
strongly stimulated by the meat juices diffusing 
through the water of the pool. All crabs meticu¬ 
lously examined the substrate with the distal 
portion of the dactyls. Moreover, all individuals 
exhibited extreme complacency when the writer 
approached sufficiently to obtain close-up photo¬ 
graphs of the group. Under normal conditions 
the similar approach of a person would instantly 
send them deep into their refuge places. It 
seemed that the stimulus to search for the 
origin of the meat juices superseded the natural 
behavior of concealment upon the approach 
of possible danger. 
The foregoing field observations suggest that 
the chemical sense is ineffective in inducing 
rapid responses, inasmuch as the animals neg¬ 
lected to search for the food until it had been 
present for several minutes. However, it seems 
likewise apparent that upon adequate chemical 
stimulation, the chemical sense is of considerable 
significance in regulating the activity of the 
crabs. It has been suggested above that moving 
food, food being eaten by other individuals, 
or food tossed sufficiently close to a crab to 
stimulate its visual centers elicits an immediate 
response. It would, therefore, seem that the 
procurance of food, both in and out of water, 
is mediated primarily through visual stimuli. 
These field data seem contrary to those set 
forth by Nagel (1894) and Bethe (1896, 
1897) for C. maenas. Both authors concur that 
this species, in its search for food, is aided by 
the chemical sense; Bethe infers that the chem¬ 
ical stimuli are the principal ones involved, the 
